Orbs, also known as Orbs of the Prophets or Tears of the Prophets, were mysterious artifacts that originated from non-corporeal entities known as the Prophets, who resided inside the Celestial Temple, otherwise known as the Bajoran wormhole.
The Orbs were central to the Bajoran religion. They appeared as hourglass-shaped energy fields; the Orbs defied conventional scientific analysis. They produced temporary but intense metaphorical hallucinations on people in the immediate vicinity, which seemed to be a form of direct or indirect communication with the so-called Prophets. (DS9: "Emissary")
As tangible objects offering communication with their gods, the Orbs were greatly treasured by the Bajoran people. The Orbs were contained inside ornate, jeweled Orb arks tended by monks, safeguarded inside monasteries and shrines. A force field also protected the Orb inside its vessel. (DS9: "Resurrection") Officially, contact with an Orb required the special permission of the Vedek Assembly, although this was a rarely followed formality. (DS9: "Emissary")
Orb experiences often provided some form of personal insight or divination into the future, and many were recorded in the form of ancient Bajoran prophecies. After an encounter with an Orb, individuals sometimes experienced continued visions known as Orb shadows. Bajorans believed that Orb shadows resulted when individuals ignored what their Orb experience had told them. (DS9: "Accession") These experiences, however, were only allowed to individuals whom the Prophets permitted to experience these visions. (DS9: "Strange Bedfellows")
In all, nine orbs were said to exist, and they appeared in the skies over the past ten thousand years; at least five were discovered in the Denorios Belt. According to Bajoran legend, they had been sent by the Prophets to provide guidance and wisdom. Eight of the Orbs were thought by Benjamin Sisko to be "in some Cardassian laboratory, being turned upside down and inside out." Even Gul Dukat thought the Cardassians had all the Orbs in their possession. (DS9: "Emissary")
As of mid-2369, one of the nine was kept on Bajor by the vedeks: the Orb of Prophecy and Change, also described as the third orb. (DS9: "Emissary", "The Circle", "The Collaborator") It was also known as the "Orb of Prophecy" or "Orb of Change". (DS9: "Trials and Tribble-ations", "Rapture", "Strange Bedfellows", DS9: "Destiny") Possession of this Orb dated back as far as the 7th century BC (DS9: "Destiny"); occasionally, it was kept at the shrine aboard Deep Space 9. (DS9: "Resurrection")
Later in 2369, Julian Bashir and Miles O'Brien learned of an Orb fragment on Bajor that was set as a jewel in a bracelet. This bracelet was kept in Faren Kag's village on Bajor, where it was wielded by the Sirah. The Sirah used this bracelet to create a creature called the Dal'Rok and focus the collective energy of the villagers to defeat it. (DS9: "The Storyteller")
In 2373, the Cardassian government contacted Benjamin Sisko and offered to return the Orb of Time to the Bajorans. Though a number of fake Orbs, in general, had cropped up over the years prior, this Orb taken from Cardassia Prime proven genuine. (DS9: "Trials and Tribble-ations") By 2374, this Orb was kept at the Temple of Iponu, on Bajor. (DS9: "Wrongs Darker Than Death or Night")
By mid-2374, knowledge of these Orbs had reached the mirror universe, where such things were not a part of the Bajoran culture. During this time, a scheme was put forth to have Bareil Antos and Intendant Kira Nerys steal the Orb of Change and return it to their Bajor in hopes of uniting the Bajorans in their war against the Klingon-Cardassian Alliance. This attempt ultimately failed. (DS9: "Resurrection")
By late 2374, the Orb of Contemplation was kept at the shrine aboard DS9 during the Gratitude Festival. Shortly afterwards, the Orb was attacked by Dukat while under the possession of the Pah-wraiths which entered the Celestial Temple through the Orb. All of the Orbs then went dark leading to widespread panic among the Bajorans, who believed that their gods had forsaken them. (DS9: "Tears of the Prophets", "Afterimage")
Three months later, in early 2375, a previously unknown tenth orb was discovered, the Orb of the Emissary. This Orb was discovered by Benjamin Sisko below a sand-covered plain on the planet Tyree. His discovery led to the restoration of all the other Orbs that had gone dark. (DS9: "Image in the Sand", "Shadows and Symbols")
In regard to why Bajorans didn't consult the Prophets more frequently, Ronald D. Moore commented, "We assume that not everyone has access to the Orbs whenever they have a question about their relationships (Shakaar was the First Minister after all, and rank hath its privileges) and also that the messages from the Orbs are often murky or confusing and may not always provide the clearest answers in matters of the heart." (AOL chat, 1997)
In the Pocket DS9 book trilogy Millennium, the Pah-wraiths built three Orbs of their own, known as the "Red Orbs of Jalbador". When these three Orbs were brought together, they opened a second Celestial Temple. However, after the Defiant travelled into the future when it went into warp around the new wormhole, a complex temporal event led to the crew managing to go back in time and disrupt the events that led to the opening of the second temple without actually changing their own experiences.
In the Deep Space Nine Relaunch novel Avatar, Book One, the Orb of Memory was found aboard a derelict Cardassian freighter in the Badlands. This Orb demonstrated the ability, when left open, to make all those aboard the freighter uncontrollably contemplate past events, to the point at which they completely lost track of their mission. When in close proximity, it also allowed Elias Vaughn to experience the memories of the dead crew of the freighter and see a vision of Benjamin Sisko. Later, in Lesser Evil, the Orbs of Destiny, Souls, Truth, and Unity were returned to the Bajoran people by a joint effort between the Cardassian Oralian Way (a religious movement) and Vedek Yevir Linjarin. All nine Orbs were later moved to Ashalla, the capital city on Bajor, in Unity.
In the miniseries Star Trek: The Q Conflict, when the four crews are charged by Q with the challenge of attracting the attention of the Prophets so that they take part in Q's current contest against Trelane, the Organians and the Metrons, Kira (currently part of Captain Kathryn Janeway's crew as part of the contestants 'trading crews') attempts to commune with an Orb to make contact with the Prophets for her 'team'.
In Star Trek Online, two more Orbs surface. Players running missions on the Cardassian Front are tasked with reclaiming one, known as the "Orb of Possibilities", from the True Way. Upon returning it to Bajor, however, the Prophets inform the player that "what has been returned does not belong," and that it in fact is from the mirror universe; the player is tasked with travelling through a rift in the Badlands to the mirror universe and then taking it to the mirror Bajor. For a limited time, upon completing "The 2800" feature episode series, the player could receive a shard of the Orb, known as the Shard of Possibilities. The second, the Orb of Peace, was found by Kai Opaka, on a moon in the Gamma Quadrant where she had been marooned since 2369 ("Battle Lines"), and was able to fulfill her goal of ending the conflict between the Ennis and Nol-Ennis trapped there with her.