(covers information from several alternate timelines)
Commander Sela was a military officer in the Romulan military, and an extremely important figure in the Empire's covert attempts to destabilize both the Federation and the Klingon Empire. She was the product of a union between Starfleet officer Tasha Yar's counterpart from an alternate timeline and a Romulan general.
Events leading to birth
In the year 2344, while in the middle of a battle with four Romulan Warbirds, the USS Enterprise-C traveled through a temporal rift to the year 2366. This had the effect of creating an alternate timeline, wherein the Federation was at war with the Klingon Empire, and Tasha Yar had not died at Vagra II.
Emerging from the rift, the Enterprise-C encountered its successor, the USS Enterprise-D. Through Guinan, the Enterprise-D crew learned that, in order for the timeline to be restored, the Enterprise-C had to return through the rift, even though doing so would almost certainly mean their deaths. Learning from Guinan that, in the other timeline, she had died a meaningless death, Tasha requested and was granted a transfer to the Enterprise-C, which returned through the rift and resumed the battle with the Romulans. (TNG: "Yesterday's Enterprise")
While the Enterprise-C was destroyed, several crewmembers survived and were taken prisoner by the Romulans, Tasha among them. After being interrogated, all of the survivors were to be executed, but a Romulan general became enamored with Tasha, and offered to spare the lives of the prisoners if she became his consort, a condition to which Tasha agreed. One year later, she gave birth to a daughter, Sela. When Sela was four years old, Tasha took her and attempted to escape, but sensing that her mother was attempting to take her away from her home and her father, Sela cried out, and Tasha was discovered and subsequently executed. (TNG: "Redemption II")
Over the next eighteen years, Sela rose through the ranks of the Romulan military, achieving the rank of Commander. In that capacity, she was a major player in plots to disrupt the alliance between the Federation and the Klingon Empire, one of which involved kidnapping Starfleet officer Geordi La Forge, and brainwashing him to kill Klingon governor Vagh. (TNG: "The Mind's Eye") She was also involved in lending support to the House of Duras in their attempt to gain control of the Klingon Empire, ferrying them materials and supplies in cloaked Romulan vessels. Sela was foiled, however, when Captain Jean-Luc Picard led Starfleet to the Klingon-Romulan border and instituted a tachyon detection grid, making it impossible for any cloaked ship to cross the border undetected. Data, in command of the USS Sutherland, managed to use the tachyon signature that formed around Sela's ships, and ordering three photon torpedoes, lit up the tachyon field around the three cloaked Romulan Warbirds, causing them to reveal themselves, forcing Sela to withdraw back to Romulan space, causing the House of Duras to lose the civil war. (TNG: "Redemption", "Redemption II")
Sela was encountered again in 2368, when she masterminded a plot to invade Vulcan, exploiting Ambassador Spock's underground reunification movement on Romulus. Using three stolen Vulcan ships carrying a Romulan invasion force, she attempted to use Spock to convince the Federation that a peace envoy was en route to Vulcan. Spock refused, however, forcing Sela to use a preprogrammed hologram of Spock as an alternative. She then made the mistake of leaving Spock alone with Picard and Data, the latter having become proficient with Romulan computer systems. When Sela returned to her captives, she was captured instead and was forced to watch as Data foiled her plot by warning the Federation, forcing the cloaked warbird escorting the stolen Vulcan ships to destroy them rather than let them be captured, before the android used a Vulcan nerve pinch on Sela, incapacitating her. (TNG: "Unification I", "Unification II")
Sela was devised and portrayed by Denise Crosby.
In devising the role, Crosby stated, "the part of Sela sort of came about from me sitting around in my house one day thinking about how much fun it was for me to go back and do "Yesterday's Enterprise", and it was so much fun that I thought what else can I do? I thought it was pretty well established that Lt. Yar and Lt. Castillo on "Yesterday's Enterprise" had something going so perhaps they had a child or Yar was pregnant when she went back into the past to fight her final battle. And I sort of thought it out and it seemed to really make sense, and there were no flaws."
"So my original intention was that Lt. Yar would have a daughter that was raised by Romulans and would grow up to try to actually be a Romulan. So I brought this up and the producers really liked the idea and they sort of toyed with it for a while. A few months went by and I got a call and they said we like your idea but we just can't make sense of that Lt. Yar got pregnant by Castillo. We'll have it so that Yar was captured, they didn't all die in the battle, the ship was captured and she was taken by a Romulan general." (Trek: The Next Generation Crew Book [page number? • edit])
In her first appearance in the episode "The Mind's Eye", Crosby only lent her voice to this character although uncredited. Actress Debra Dilley appeared as the photo double standing in the shadows. She was identified as "Unseen Woman" in character section the episode's script, and noted as being one of "several shadowy figures [who] might be A WOMAN, but it's hard to be sure," it is further noted when she speaks, she has "a questioning, authoritative tone in her voice." The only other revelation about the character is her being addressed as "Commander". 
In her second appearance, she was identified as "Mysterious Woman" in the character section of the episode's script, and presented much the same way in the script notes, her as "ANOTHER FIGURE sitting in he darkness [..] who might be a woman, but we're not sure yet," which further notes, "The Woman in shadow doesn't have to raise her voice in order to command everyone's attention instantly. There is something deadly in her calm voice and assured manner." In the moments leading up to her reveal, the script notes that "the Mysterious Woman heard from earlier is sitting in a chair with her back to us," and "the Woman turns around in her chair so that her features become apparent for the first time. We're shocked as we see that she's Human... with blonde hair... and that she bears a striking resemblance to the late Tasha Yar." 
In "Redemption II", Cameron served as hand double for Crosby's character. According to the script, her name was pronounced SEE-lah, and it was said "Sela's attitude is crisp, dispassionate... very Romulan. B'Etor is very deferential to Sela... she clearly fears her." Her personality is further explored after she enters the conference room aboard the Enterprise, where the script notes described that "Sela immediately strides into the room, looking around, pacing like a jungle cat, covering the room as though marking territory. She is lithe, genial, confident -- and definitely going for the upper hand." The script notes continue, through the continuation of their interaction, that she slowly loses her upper hand to the challenges made by Picard.  Her character is further addressed as "Commander", and in fact, she even appears to given orders to the seemingly senior officer, General Movar, on nearly all their interactions.
Though Sela states how Tasha, her mother, died when she was four for trying to escape, Picard is completely unconvinced that her story is true, and insists it won't affect his judgment in any way. Sela's story is supported by Guinan, who retains memories from the alternate timeline shown in "Yesterday's Enterprise".
Sela returned to plague Captain Jean-Luc Picard and company several more times in the Star Trek novels, notably in Peter David's books Triangle: Imzadi II, in which she attempted to trick Thomas Riker into releasing a nanovirus that would destroy the entire Klingon population on their homeworld; the plan was thwarted by the intervention of William T. Riker, Worf and Odo. Another notable return was in book five of the Double Helix series Double or Nothing. In the book The Romulan Stratagem, Sela forms an alliance with Data to uncover the truth about a series of disastrous events that cast suspicion on both the Romulans and the Federation. She later appears in Michael Jan Friedman's TNG relaunch novel Death in Winter, as an agent of the Romulan government formed by the newly ascended Praetor Tal'aura, whose other allies include Tomalak. During the Typhon Pact series, Sela has become the head of the Tal Shiar, mounting various operations to try and steal the technology for the Quantum slipstream drive from the Federation. After her actions result in the destruction of Deep Space 9 and the Federation and the Pact nearly being pushed to war in the novel Raise the Dawn, Sela is arrested by the current Praetor – who notes that Sela's desire to ensure Romulan supremacy have actually put them in more danger than if she had done nothing – and Sela commits suicide rather than allow herself to be handed over to the Federation for trial.
The novelization of Unification gives the name of the Romulan general who fathers Sela as "Meldet".
Sela also appeared, with the rank of Admiral, in the computer game Star Trek: Armada. Further, in the comic book miniseries Star Trek: The Next Generation - The Killing Shadows by Scott Ciencin and Andrew Currie, Sela resurfaces as an exile from Romulus, serving alongside Picard on the planet Nydaris in common cause against a race of ninja-like assassins called the Bodai Shin.
In Star Trek Online, Sela became Proconsul in 2384, but was exiled from Romulus after losing a power struggle with the Tal Shiar. As a result, she survived the Hobus supernova of 2387 that destroyed Romulus and killed many of her political opponents. In 2403, she becomes Praetor, and five years later reforms the Empire into a monarchy, with herself as its Empress. In the game itself, the player encounters Sela in the mission "Cutting the Cord," where the player must fight her personal Scimitar-class warbird, the IRW Leahval. Before the player can destroy it, an Iconian ship arrives from an Iconian gateway and tows her ship to an unknown location. Her role was greatly expanded in the May 2013 Legacy of Romulus expansion pack, where she serves as a secondary villain in the Romulan Republic storyline (the primary antagonist being Colonel Hakeev of the Tal Shiar).
In the October 2014 Delta Rising expansion, Sela is found in an Elachi installation hidden in the Delta Quadrant, and arrested by the Romulan Republic. However, she escapes, and solicits the aid of the player character to find the Iconians; they travel first to Hakeev's abandoned base in the Nopada system, and then to a Dyson sphere in the Andromeda Galaxy, where they encounter the renegade Praetor Taris and discover a massive Iconian invasion fleet. The sphere then shifts to orbit around Iconia itself, and Sela remains behind while the players escape. A series of subspace messages to the player reveal that she is attempting to gather Dominion support against the Iconians.
During the final battle of the Iconian War at Earth, Sela returns with Dominion reinforcements in order to buy time for a time travel mission to ancient Iconia using a Krenim weapon ship. In retaliation for the destruction of Romulus, Sela kills several Iconians as they flee through gateways, causing the Iconians to swear vengeance on the Romulans. This leads to a predestination paradox: Sela, in her desire to avenge the destruction of Romulus, was in fact the reason it occurred.
Sela returns in the mission "Survivor", where the player character hunts her down at the suggestion of Agent Daniels to help search for the alternate timeline version of Admiral T'Nae. Sela agrees, understanding she's effectively under arrest. During their search, they find datapads revealing that her mother was never killed, instead left with the rest of the crew of the Enterprise-C on a prison colony, shaking Sela greatly. Over the course of the mission, Sela learns that Yar always loved Sela and wished she could see her, sadly being unable to as she died of natural causes before the player could arrive. After being put under arrest, Sela asks an unknown Starfleet officer (implied to be Data, as he shows his holoimage of Tasha to her, though his face is not seen) to tell her more about Tasha.