The Borg Queen was the name given to the entity that existed within and served as a central nexus for the Borg Collective. The name was given to her by the Federation scientists Magnus and Erin Hansen, who discovered her existence. (VOY: "Dark Frontier")
The Queen defined herself as: "I am the beginning, the end, the one who is many. I am the Borg." Although this suggested she was an individual within the Collective, while addressing Borg drones as "my drones", she was not an individual. The purpose of the Queen was to bring order to chaos. (Star Trek: First Contact)
The Borg Queen was a humanoid female of Species 125. In accordance with the Borg pursuit of perfection, a blending of the organic and synthetic, very little of her original humanoid form remained. Her face and upper torso were organic while the rest of her body, including her skull and spinal cord, were synthetic. Because of her disembodiment she saw herself as the epitome of perfection. The Queen had her own chambers within the Borg Unicomplex from which she could oversee and control the Borg via the command interface. Whether she had her own ship or not is unknown, but she used different Borg vessels to travel, such as a Borg cube, sphere, or diamond. When her physical presence was not necessary her organic part resided above this chamber while her synthetic parts were stored below it, under the floor. If she desired to do so, both could be brought together, and in doing so, created a humanoid form for herself.
In Human terms, the Borg Queen could be characterized as ruthless. She would do anything to protect the Borg Collective. Where drones showed no emotions, the Queen herself did. When necessary, she would employ psychological tactics, like extortion or plain intimidation to get what she wanted. The Queen even displayed self-preservation when she was held at gunpoint by Captain Janeway, who threatened to kill her. On a personal level, she considered Seven of Nine her favorite drone, because the Queen considered her to be unique. This had to do with Seven's experiences as an individual when she served aboard USS Voyager. The Queen's goal was to re-assimilate Seven into the Collective to enhance the Borg's, and so her own, perfection. She launched a Borg Diamond and followed the Delta Flyer into a Transwarp conduit. The attempt failed, but she survived. (VOY: "Unimatrix Zero", "Dark Frontier")
The death of a Borg Queen did not seem to affect the Collective or its Hive mind. When a Borg drone died, its memories would still be within the Hive mind. Borg drones were capable of functioning without a Queen for any length of time by forming a Hive mind of their own. (VOY: "Unity")
It was thought by Federation exobiologist Erin Hansen, that the Borg Queen functioned like the queen of an insect hive, to coordinate the drones. Evidence of this was later seen when the Queen countermanded the Collective's judgment about assimilating Voyager. While the Collective felt that assimilation was warranted, the Borg Queen countermanded them and justified the decision due to the fact that Voyager didn't compromise their security. (VOY: "Endgame")
The Queen, while providing coordination for the drones she commanded, also provided other functions such as regulation of the Borg's Transwarp hubs and Interspatial manifolds. She effectively brought "order to chaos" for all things. (VOY: "Endgame")
The existence of a Borg Queen was first established sometime prior to 2365 by the exobiologists Erin and Magnus Hansen. However, because the Hansens were assimilated, their discovery never reached the Federation. (VOY: "The Raven")
It was not until 2373, that the Federation became aware of her when the Federation starship USS Enterprise-E prevented the assimilation of Earth. This was the second attempt by the Borg, also known as the Battle of Sector 001. The Borg Queen, along with other drones, traveled back to Earth's past to prevent First Contact, and by doing so, hoped to be able to assimilate Earth.
During this conflict, while Captain Jean-Luc Picard was trying to destroy the Borg, the Queen claimed to have been present during the Battle of Wolf 359, and even admitted that Locutus of Borg – the assimilated Picard – was intended to be a singular intelligence – her counterpart, was intended to ease the burden of loneliness. However, when Picard continued to resist, even when he could not control Locutus' body, she was regretfully forced to turn him into the form in which Starfleet encountered him, a glorified drone. Whether or not she physically took part in the Battle of Wolf 359 was unknown.
Picard killed that Borg Queen after she tried to persuade Lieutenant Commander Data to give her the encryption code by which he had locked the Enterprise's computer. She ordered Data to destroy the Phoenix spaceship with quantum torpedoes, and taunted Picard that she would rule Earth without Humans or the Federation in it, when the torpedoes missed. Data told her, "Resistance is futile!" and vented the warp core plasma coolant, which destroyed her organic parts. Picard then broke her cybernetic spinal cord, which ensured that she could no longer function. (Star Trek: First Contact)
The second encounter with a Borg Queen was in 2375 in the Delta Quadrant. Here, the lost Federation starship USS Voyager, tried to rescue the former Borg Drone, Seven of Nine, who was then part of Voyager's crew, when the plan to steal a transwarp coil from a Borg sphere did not work out as planned. The Queen also revealed that Seven of Nine was not really freed by Voyager from the Collective, but was allowed to leave by the Borg. During this encounter, the Borg Queen hoped to assimilate Seven of Nine again, who experienced life as an individual for two years, and by doing so, add to her own perfection. However, Seven rejected the Queen and fled with a rescue mission sent by Voyager in the Delta Flyer. The Borg Queen's Diamond was sent by the Queen to intercept the shuttle, but it was destroyed in the attempt. (VOY: "Dark Frontier")
In 2377, a Borg Queen was again encountered by Voyager. This time the Queen wanted to destroy Unimatrix Zero, a virtual world that was populated by regenerating Borg with a genetic mutation. This world was discovered by Seven of Nine and posed a threat to the Borg. During Voyager's efforts to rescue this virtual world, the Borg Queen demonstrated her powers by destroying a Borg sphere because she could no longer "hear" only one drone. When a nanovirus was released to prevent the detection of Unimatrix Zero, the Queen destroyed several Borg vessels, and killed 75,000 Borg Drones in the process, in the hope of persuading the captured Captain Janeway to give her the antidote. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")The last encounter between a Federation starship and a Borg Queen was in 2378, and again Voyager played a part in it. Voyager accidentally discovered a Borg transwarp hub within a nebula and were helped by Admiral Kathryn Janeway, who came from an alternate timeline around twenty-six years in the future, to use the Borg transwarp network to get back to the Alpha Quadrant. Because the Borg guarded their Transwarp hub closely, Admiral Janeway devised a plan by which she would infect the Borg Queen with a neurolytic pathogen and in doing so make her lose control over the force fields which protected the interspatial manifolds. When the admiral was captured by the Borg, near the Unicomplex, she was assimilated by the Borg Queen herself. Soon after, the Queen began to lose control over her drones. The pathogen even made her lose control over her own synthetic parts, as her body literally fell apart. Her death caused the destruction of the Unicomplex and despite her efforts, Voyager reached Earth safely. The Borg sphere that was sent after them by the Queen was destroyed by Voyager's transphasic torpedoes, which were given to them by Admiral Janeway from the future. (VOY: "Endgame")
- The Borg Queen was played by Alice Krige in Star Trek: First Contact and VOY: "Endgame". The character was played by Susanna Thompson in the I AM ERROR episodes "Dark Frontier", "Unimatrix Zero", and "Unimatrix Zero, Part II".
- In an early design meeting for the Borg Queen, the movie Captain EO was mentioned, regarding Anjelica Huston's performance as a villainous woman who lived in the ceiling and would descend on cables. 
- The appearance of the Borg Queen in First Contact was a controversial one in the Star Trek universe. Though the Borg provided for a threatening and intriguing alien enemy, their lack of a single villain presented a challenge for the writers. To counter this, and to expand some on the original notion of the Borg as an insect-hive type of race, they created the Borg Queen as a focal point for their story. Writer Brannon Braga has stated in this respect, "I think some people liked the Borg Queen and some didn't, but to us the Borg Queen was the thing that made it all work. We realized very quickly that the Borg aren't that interesting for a feature film for two hours because they don't say anything. They're robot zombies. So, to me, the Borg Queen was the coolest new thing about that movie." 
- Consequently, many different theories have developed over the role of the Queen and the extent to which she may have represented a hierarchical structure in the previously supposed-to-be Collective nature of the Borg. The exact nature of the Queen is still hotly debated and has many possible explanations. When Data asked her about this in First Contact, she openly denied simply controlling the Collective, and said that she was the Collective, and claimed to "bring order to chaos" when Data tried to get a clearer explanation. He considered her response "interesting, if cryptic."
- Later in First Contact, when asked by Picard how she had survived when the cube that was sent to Earth in 2367 was destroyed, the Queen only replied that Picard had become small, and thought in three-dimensional terms. Exactly what was meant by this was also a subject of speculation.
- This was further complicated by the re-appearance of a Queen during the run of Voyager, and her subsequent "deaths" and reappearances during the show's run, but was not directly addressed.
- Alice Krige purposely limited the ways in which she prepared for "Endgame", reviewing neither her own work on First Contact nor any of Susanna Thompson's portrayal of the same character. This choice was not motivated out of any sort of disrespect for Thompson, and had nothing at all to do with the actress. (Star Trek Magazine issue 169, p. 52; ) Krige speculated, "Whoever had played the role, I would have made the same decision."  Explaining why she made the choice, Krige conceded, "I thought to see someone else's performance would throw me off course. It was already going to be fairly different because it was the Borg Queen with two females, as opposed to the Borg Queen with two males [...] I just felt it wouldn't help the process." (Star Trek Magazine issue 169, p. 52) She also related, "I didn't want something in my head, in my imagination. I needed my performance to happen in the moment."  Krige did, however, request to receive and read all the Voyager scripts featuring the Borg Queen, including the new teleplay for "Endgame". She indeed read the scripts, despite not watching any of the episodes. (Star Trek Magazine issue 169, p. 52; )
- In 2002, the Borg Queen was placed second in TV Zone's list of the top twenty science fiction television villains. Dukat was fourth, Weyoun was eighth, Q was eleventh, and Seska was nineteenth.
- In an early version of the script of Star Trek: First Contact (a script very different from the movie), Geordi La Forge tells Data that he is sending the Borg Queen's remains to the Daystrom Institute for study. 
- When asked whether the Queen was a "virtual entity; the personification of the collective", Braga's writing partner, Ronald D. Moore, said, "This was not the intention. We saw her as a literal person." (AOL chat, 1997)
- According to Judith and Garfield Reeves-Stevens, they pitched a story for an episode of I AM ERROR where Alice Krige would play a Starfleet medical technician who made contact with the Borg from "Regeneration". The encounter would have been the birth of the Borg Queen. 
- An undersuit that was worn by Krige in First Contact was sold off as lot 9677 in the It's A Wrap! sale and auction on eBay.
- The Borg Queen, Dimitri Valtane, Lojur and Admiral Hayes are the only characters to debut in a Star Trek film before appearing in a Star Trek television series.
According to the Pocket VOY novel, The Farther Shore published after the television series concluded, a Borg Queen could be replaced in mere seconds by using the Royal Protocol. Seven of Nine was specifically mentioned in the Royal Protocol and was most likely to become the next Queen.
The Pocket TNG novel Resistance showed the creation of another Queen, who was destroyed by the crew of the Enterprise-E. Subsequently, in the Pocket TNG novel Before Dishonor, Admiral Janeway was assimilated by the Borg and became a Queen who was eventually defeated by Seven of Nine.
In the Star Trek: Destiny trilogy, a newly installed Queen oversaw a massive Borg invasion of the Alpha Quadrant. It was later revealed that the Borg Queen was merely an avatar for the true power behind the Collective. The Destiny trilogy also mentioned that multiple Queens have been known to exist simultaneously in the Collective, but they all possessed the same agenda.
One theory regarding the creation of a Queen is that "queens" are members of a specific race, one that was chosen because its females exhibited superior higher-order brain processing-speed, and were therefore assimilated and bred for that purpose. (Star Trek: Elite Force II; Star Trek: Legacy)
The extra section of the game Star Trek: Legacy contained the "Origin of the Borg", which told the story of V'ger being sucked into a black hole. V'ger was found by a race of living machines which gave it a form suitable to fulfilling its simplistic programming. Unable to determine who its creator could be, the probe declared all carbon-based life an infestation of the creator's universe, leading to assimilation. From this, the Borg were created, as extensions of V'ger's purpose. Drones were made from those assimilated and merged into a collective consciousness. The Borg Queen was created out of the necessity for a single unifying voice. However, with thoughts and desires of her own, she was no longer bound to serve V'ger. This explanation, however, was not canon.
According to "Shinsei Shinsei", the Borg Queen's name was Danzek.
Borg Invasion 4D
In 2004, the Borg Queen made a re-entry onto the big screen when the Borg Invasion 4D-ride premiered at the Star Trek: The Experience, an interactive attraction that incorporated live-action stage performance and animation, in which the visitors had a limited part themselves, within a 3D cinema environment. The movie for the attraction was mostly produced by the veteran Star Trek production team on the studio's own premises.
The storyline, set after the events depicted in "Endgame" entailed yet another incursion into Federation space by a Borg cube, attacking Copernicus Station and capturing a shuttle with its occupants (the attraction visitors), who were trying to escape from the overran space station. While the captured crew was being prepared for assimilation, the Queen made a dramatic entrance and, true to form, begins lecturing about the perfection of the Borg Collective and demanded the surrender of the group's inhibitions and instructed them to join the hive mind. When all seemed lost and much to the dismay of the Queen, Admiral Janeway came to the rescue, by flying USS Voyager straight into the cube, destroying the tractor beam that held the shuttle, enabling it to escape, in the process inflicting critical damage to the cube, which subsequently blew up. Again true to form, the Queen made her escape, but not before exclaiming, "Savor your victory! We will meet again!"
For the film portion of the ride, some of the original, former Voyager cast reprised their respective roles, including Alice Krige as the Queen. Many of the Borg featured in the film (as opposed to the attraction live crew performing as such), were played by performers who had already done so for First Contact (or for the respective Voyager television episodes); "It was a most joyful reunion," Krige declared tongue-in-cheek. When presented with the first 3D footage of her close-up scenes, Krige admitted to being flabbergasted by her own, literal in-your-face performance. (VOY Season 7 DVD-special feature, "The Making of Borg Invasion 4D") While an official Star Trek franchise production, events depicted in the film are, as usual for these kind of productions, not considered canon, and treated as apocrypha.