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 Borg Queen was assimilated to the Collective around the age of 7-8 and was already present in 2354. (VOY: "Unimatrix Zero, Part II") 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.
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 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 create 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 her vessel 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 justifying 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, 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 is 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 photon torpedoes and taunted Picard that she will rule the earth without humans or the Federation in it when, the torpedoes miss. Angry at Data for the failure to take over Earth, the android tells her "Resistance is futile!". Warp core plasma coolant released by Data destroyed her organic parts, after which Picard broke her cybernetic spinal cord, ensuring 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 the drone Seven of Nine was not really freed by Voyager from the Collective, but 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 so add to her own perfection. However, Seven rejected the Queen and fled with a rescue mission sent by the Voyager in the Delta Flyer. A Borg vessel 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 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, killing 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-three 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 making 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 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 sent after them by the Queen was destroyed by Voyager's transphasic torpedoes, given to them by the admiral 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 Voyager 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 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 Queen as a focal point for their story.
- Consequently, many different theories have developed over the role of the Queen and the extent to which she may represent 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, claiming 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 sent to Earth in 2367 was destroyed, the Queen only replied that Picard had become small, thinking in three-dimensional terms. Exactly what was meant by this is 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.
- 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", Ronald D. Moore said, "This was not the intention. We saw her as a literal person." (AOL chat, 1997)
- According to Judith Reeves-Stevens and Garfield Reeves-Stevens, they pitched a story for an episode of Star Trek: Enterprise where Alice Krige would play a Starfleet medical technician who made contact with the Borg from "Regeneration". The encounter would have seen the birth of the Borg Queen. 
- An undersuit worn by Krige in First Contact was sold off on the It's A Wrap! sale and auction on eBay. 
According to the Pocket VOY novel The Farther Shore published after the television series concluded, a Borg Queen can 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 is assimilated by the Borg and becomes a Queen who is eventually defeated by Seven of Nine.
In the Star Trek: Destiny trilogy, a newly installed Queen oversees a massive Borg invasion of the Alpha Quadrant. It is later revealed that the Borg Queen is merely an avatar for the true power behind the Collective. The Destiny trilogy also mentions that multiple Queens have been known to exist simultaneously in the Collective, but they all possess 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 exhibit superior higher-order brain processing-speed, and are therefore assimilated and bred for that purpose. (Star Trek: Elite Force II; Star Trek: Legacy)
The extra section of the game Star Trek: Legacy contains the "Origin of the Borg", which tells 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, is not canon.
According to "Shinsei Shinsei", the Borg Queen's name is Danzek.