From UFStarfleet LCARS
The Borg are a pseudo-race of cybernetic organisms. They use abduction and "assimilation" (enforced cybernetic enhancement, connection to the hive mind) as a means of "achieving perfection".
Aside from being the main threat in the Delta Quadrant, the Borg play major roles in other quadrants, primarily as an invasion threat to the United Federation of Planets, and serve as the way home to the Alpha Quadrant for isolated Federation starship Voyager. The Borg have become a symbol in popular culture for any juggernaut against which "resistance is futile".
The Borg manifest as cybernetically-enhanced humanoid drones of multiple species, organized as an interconnected collective, the decisions of which are made by a hive mind, linked by subspace radio frequencies. The Borg inhabit a vast region of space in the Delta Quadrant of the galaxy, possessing millions of vessels. They operate solely toward the fulfilling of one purpose: to "add the biological and technological distinctiveness of other species to [their] own" in pursuit of "perfection." The concept of perfection is the unifying idea at the core of the Borg. The pursuit of an unemotional, mechanical perfection is the Borg's only motivation. This is achieved through forced assimilation, a process which takes individuals and technology, enhancing and controlling them.
Captain Jean Luc Picard of the USS Enterprise - D is abducted and subsequently assimilated by the Borg and transformed into Locutus, the Latin term for "he who has spoken". "Locutus" is the Borg method of describing the former Picard as the representative of the Borg in all future contacts related to humanity. Picard's knowledge of Starfleet is gained by the collective, and the single cube easily wipes out all resistance in its path, notably the entire Starfleet armada at Wolf 359, which consisted of 40 starships, some of which were sent from the Klingon Empire. The Enterprise crew manages to capture Locutus and gain information through him which allows them to destroy the cube. Picard is later "deassimilated".
In the Enterprise - E Mission "First Contact" it is recorded that The Borg return as antagonists. After again failing to assimilate Earth by means of a direct assault in the year 2373, the Borg travel back in time to the year 2063 to try to stop Zefram Cochrane's first contact with the Vulcans, which would have erased the Federation from history. However, the Enterprise-E follows the Borg back in time and the crew restores the original timeline. First Contact introduces the Borg Queen.
Captain Kathryn Janeway of the USS Voyager has had frequent encounters with the borg which is documented in Starfleet Records.
United Federation Starfleet, established its base in the Delta Quadrant based on Voyager's data. With the borg a primary threat to the known universe it was deemed a colony at Pinastri would be established to monitor any and all activity in the Delta Quadrant.
Though Borg rarely look alike, they share several common characteristics. Borg commonly have one eye (most often the left eye) replaced with a sophisticated ocular implant which allows them to see beyond the human visual spectrum. This implant usually projects a red laser beam. They also usually have one arm replaced with a multi-purpose tool and flat white skin, giving them an almost zombie-like appearance.
Owing to their cybernetic enhancements, all Borg are far stronger than humans to varying degrees (depending on the species the drone came from). However, they never run to their destination, and hence most species can outpace them. Borg drones are resistant to phaser fire, being completely immune to the stun setting. In addition, all Borg drones possess personal shielding which collectively adapts to phaser fire. In various missions, phasers tend to become ineffective after a dozen shots at most, depending on the settings and time between shots.
Phaser frequencies can be altered to penetrate the shield, but the Borg adapt more quickly with each modulation. Due to this, crews have been known to employ a variety of other countermeasures such as holodeck-generated bullets and melee weapons, as demonstrated in the mission First Contact.
The Borg hive mind can lead to certain downsides: Borg drones have a weakness in that they will usually ignore anything which does not present itself as a direct threat (unless specifically directed to attack), allowing armed but passive crew to walk among them relatively unscathed until threatening behavior was observed.
The most important cybernetic component of any Borg is their "cortical node", which controls every other implanted "fixed location" cybernetic device within a Borg's body, and is most often implanted in the forehead above the usually-retained organic right eye. If the cortical node fails, the afflicted individual Borg eventually dies, as it cannot be replicated or repaired. However, successful replacement of the node can be carried out on a Borg vessel if the failure is detected promptly before the afflicted Borg's impending death.
Also referred to as the "hive mind" or "collective consciousness", this is the term used to describe the group mind of the Borg civilization. Each Borg individual, or drone, is linked to the collective by a sophisticated subspace network that ensures each member is given constant supervision and guidance. The collective is broadcast over a subspace domain similar to that utilized by the transporter.
Being part of the collective offers significant biomedical advantages to the individual drones. The mental energy of the group consciousness can help an injured or damaged drone heal or regenerate damaged body parts or technology. The collective consciousness not only gives them the ability to "share the same thoughts", but also to adapt with great speed to defensive tactics used against them