Excerpt from Memory Alpha:
Seven of Nine (full Borg designation: Seven of Nine, Tertiary Adjunct of Unimatrix 01) was a Human female who was a former Borg drone. She was born Annika Hansen on stardate 25479 (2350), the daughter of eccentric exobiologists Magnus and Erin Hansen. She was assimilated by the Borg in 2356 at age six, along with her parents, but was liberated by the crew of the USS Voyager in 2374. She joined the crew and returned to the Alpha Quadrant with the starship in 2378.
Magnus and Erin Hansen, Annika's parents, were exobiologists investigating the existence of the Borg. After a great deal of persuasion, the Federation granted the Hansens the use of the USS Raven, a small long range craft, to aid them in their investigation. In 2353, they took Annika, then aged four, along with them. They spent a good deal of time aboard the Raven in search of the Borg; Annika celebrated three birthdays aboard the ship. Eventually, the Hansens encountered a Borg cube and followed it through its transwarp conduit into the Delta Quadrant, the Borg's region of origin. They gathered a great deal of scientific data on the biology of Borg drones and the nature of the Collective, moving undetected through Borg space due to multi-adaptive shielding, invented by Magnus Hansen. They even went aboard Borg vessels, using bio-dampeners to remain undetected. However, their research came to an abrupt end in 2356 when an ion storm struck the Raven. The ship sustained damage, including, most importantly, damage to the multi-adaptive shielding, which went off-line for 13.2 seconds. This left them exposed long enough for the Borg to detect them and perceive them as a target for assimilation. The Hansens tried to evade pursuit by masking the Raven's warp trail, but the Borg found them. They and their daughter were promptly captured and assimilated. (VOY: "The Raven", "Dark Frontier")
In early 2368, Seven of Nine, along with three other drones in her unimatrix, crash-landed on a planet in the Delta Quadrant. The other drones, who were assimilated as adults, began to regain their identities upon being severed from the Borg Collective, but Seven was frightened as she knew nothing else but life as a drone. She forcibly linked the other drones together into a temporary collective in order to suppress their identities, and they were retrieved soon after. (VOY: "Survival Instinct") Seven of Nine remained a Borg drone until 2374, when she was liberated.
The transition back to Humanity was difficult for Seven of Nine. She appeared to accept her severance from the Collective, but tried to contact it at the first available opportunity. She was stopped, however. The Doctor, Voyager's holographic Chief Medical Officer, was able to remove most of her implants and restore most of her human appearance, but she still had some Borg technology left; these were tied into her vital functions, and removing them would kill her. Also, she still needed to regenerate, like any Borg drone, using a Borg alcove. She was given a special uniform and also issued a combadge. (VOY: "The Gift") However, she did not want to begin using her real name, Annika Hansen; Seven of Nine was the designation she had always known. But she accepted a shortened version, 'Seven' at the suggestion of Captain Janeway. (VOY: "Day of Honor")
As a former drone, Seven had considerable superior physical characteristics over most humans. Her visual acuity was vastly superior, due to her ocular implant, she possessed an eidetic memory and had superior physical strength for a healthy Human woman her age. (VOY: "The Gift", "Vis à Vis", "Tsunkatse", "Scientific Method", "Relativity") She was also much more resistant to injury and was resistant to many forms of radiation, including chroniton (VOY: "Year of Hell") and subnucleonic, that would quickly kill an ordinary Human. (VOY: "One") In addition, she became an extremely proficient martial artist, becoming a master of the Norcadian martial art of Tsunkatse. (VOY: "Tsunkatse")
Along with Ensign Harry Kim, Seven of Nine designed and constructed the ship's astrometrics lab, which used Borg technology to plot routes that trimmed several years off of Voyager's journey. The lab became an important asset to Voyager and was Seven's domain for the rest of the journey. (VOY: "Revulsion", "Year of Hell") Using the astrometrics lab, Seven discovered the Hirogen communications network, which allowed Voyager to temporarily receive messages from the Alpha Quadrant. (VOY: "Hunters") When monthly data streams and, later, realtime communication became possible, Seven helped implement enhancements to Voyager's deflector dish. (VOY: "Life Line", "Author, Author")
When Voyager crossed the territory of the anti-telepathic Devore in 2375, Seven helped develop a transporter suspension technique in order to hide telepathic crewmembers and Brenari refugees.
In 2376, Seven developed enhancements to her alcove that allowed her to process information and make connections between various events while she regenerated. This allowed Seven to deduce that photonic fleas had been degrading sensor efficiency and that a graviton catapult built by an alien named Tash employed a tetryon reactor like that of the Caretaker's array. However, the process overloaded her cortical implants, and she began drawing wild conclusions concerning Voyager's mission and crew. She convinced Chakotay that Voyager's presence in the Delta Quadrant was intentional and a prelude to a joint Federation/Cardassian invasion, while later telling Captain Janeway that Chakotay was organizing a Maquis rebellion, using technology of the Caretaker, to launch strikes on Federation and Cardassian targets. She then began to believe that Voyager was sent to the Delta Quadrant to retrieve her from the Borg Collective, and that she would be analyzed and dissected upon return to the Alpha Quadrant. She left the ship in the Delta Flyer, but Janeway was able to convince her that this 'conspiracy' was only a delusion caused by her alcove enhancements. She returned to Voyager and the enhancements were removed. (VOY: "The Voyager Conspiracy")
For More, see the article "Seven of Nine" on Memory Alpha.