Seven of Nine was born as Annika Hansen in 2348 at the federation Tendara Colony, Stardate 25479. In the few years of having a free and happy childhood she dreamt about becoming a ballerina. Her parents, Magnus and Erin Hansen both were scientists,  exobiologists. On Stardate 32611 the family took off in the U.S.S. Raven from Deep Space 4 to study the Borg, a cybernetic life-form from  the Delta Quadrant. They finally detected a Borg vessel, a Cube, studied them for several months and even managed to board the Cube for a period, undetected. But luck ran out and in the end the Raven was attacked in the B'omar region and crashed on a moon where it was boarded by the Borg. There,  at the age of six, Annika Hansen was forced into becoming a Borg drone. Perhaps the first human drone ever. Magnus was assimilated too. The fate of Erin is unknown.  (for info about assimilation see bottom of this page) The U.S.S. Raven was assimilated too and a homing device was left which started emitting a signal.

About 20 years later in 2374 Annika, then called Seven of Nine, tertiary adjunct of uni-matrix zero one, was assigned to the Starship U.S.S. Voyager. She would be a liaison between the Starship and the Borg Cube when both races had agreed to form an alliance against a new and deadly mutual enemy, Species 8472, that had launched an attack against all life the previous year. When the war with Species 8472 was over the Borg broke all agreements and tried to overtake the U.S.S. Voyager by assimilating the bridge officers through their drone Seven of Nine. But the captain of the Voyager, Kathryn Janeway,  had anticipated this and they managed to break the link from Seven of Nine with the Borg. Being cut off from the Borg Collective all her systems fell back on the single human body she had an it's myriads of Borg implants that could not function properly now. In sickbay 82 percent of the Borg implants were removed from. The remaining Borg technology could not be removed without damaging the human body. Her left eyepiece was replaced by an artificial eye. 
When Seven of Nine regained consciousness she was frightened and furious and it took long time before she was willing and able to adapt to a human community.  She didn't wanted to be called Annika again and not being part of the Nine anymore she agreed to be called Seven from then on.

Completely breaking the links from the former drone Seven of Nine with the Borg collective proved to be impossible. The homing device planted on the U.S.S. Raven by the Borg eventually effected the mind of Seven and she returned to the crash site. She and Tuvok who was with her, almost died in the wreck when it was fired upon but they escaped just in time. Before the Raven was completely destroyed Voyager managed to save much of the work her parents had done studying the Borg.
Apart from this receiving signals from Borg origin, Seven also kept transmitting data back to the Borg, even after the Borg implants were stripped off here body. Some of the implants, including her  eyepiece were the cause of that. This came to light on Stardate 51893.5 when the wreck of a Borg vessel was found on an asteroid. When examined, Federation scientists retrieved a file called Unimatrix Zero One, which held the date Seven's active implants had been transmitting since she had boarded Voyager. The Starship had been out of range and contact with the Federation almost since the beginning of it's journey and through this data unknown facts about Voyager and it's crew and the Borg were revealed to the Federation.

Seven had another encounter with her past when, in trying to steal a transwarp conduit from a Borg vessel, she was drawn to the Borg Queen. There she met her assimilated father. The Borg ship was destroyed in the end.

Origin: Delta Quadrant. A cybernetic life-form thousands of years old which is part organic, part artificial life. They have advanced well beyond Federation science, unknown prior to a confrontation with the U.S.S. Enterprise NCC-1701-D arranged by the entity known as "Q" circa stardate 42761 ( 2365) some 7,000 light-years past explored Federation space in the Delta Quadrant.

The Borg have a singular goal, namely the consumption of technology, rather than wealth or political expansion as most species seek. According to their spokesman, in the form of an assimilated Captain Jean-Luc Picard, the Borg only want to "raise the quality of life" of the species they "assimilate." Androids, for example, they view as primitive and obsolete. Born humanoid, they are almost immediately implanted with bio-chips that link their brains to a collective consciousness via a unique subspace frequency emitted by each drone. This collective consciousness is experienced by the Borg as "thousands" of voices — they are collectively aware, but not aware of themselves as separate individuals. Consequently, they never speak in singular pronouns, referring to themselves when required as merely "Third of Five," for instance.

The Borg ingest only energy to drive their technological system via an energy conduit port. Their bio-chips synthesize any organic nutrients needed. Among the many advantages their collective consciousness affords them, the Borg hive-mind allows for instantaneous adaptations to shield and phaser frequency modulations in combat; they are also able to regenerate and repair their massive cube ship with the power of their collective thoughts alone. The hive-mind drones do not register as individual life-signs when scanned, only as a mass reading and then at a bare minimum. The sick and injured are not healed but "reabsorbed" by the removal of the receiver piece, which leads to self-destructive dissolve. When shipboard during dormancy in their regenerative mode, power is minimal and the vessel's EM field cuts off. They have a knock-out drug or procedure for humans, using a drill-tap placed behind the left ear, that works immediately but wears off in less than a minute.

Abstract taken from Federation Library Database at Startrek.com