The Borg are a fictional pseudo-race of cybernetic organisms depicted in the Star Trek universe, first encountered when Q brings the Enterprise to meet them in 2365. Use this tag with the [star-trek] tag.
Whereas cybernetics are used by other races in the science fiction world (and in recent times the real world) to repair bodily damage and birth defects, the Borg voluntarily submit to cybernetic enhancement as a means of achieving what they believe to be perfection (they also force their idea of perfection on others).
Aside from being the main threat in Star Trek: First Contact, the Borg also play major roles in The Next Generation and Voyager television series, primarily as an invasion threat to the United Federation of Planets and the means of return to the Alpha Quadrant for isolated Federation starship Voyager, respectively. 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 to subspace domain. The Borg inhabit a vast region of space in the Delta Quadrant of the galaxy, possessing millions of vessels and having conquered thousands of systems. 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. This is achieved through forced assimilation, a process which transforms individuals and technology into Borg, enhancing - and simultaneously controlling - individuals by implanting or appending synthetic components.
In their first introduction to the franchise (Q Who?), little information is forthcoming about the Borg or their origins and intents. In alien encounters, they exhibit no desire for negotiation or reason, only to assimilate. Exhibiting a rapid adaptability to any situation or threat, with encounters characterized by the matter-of-fact statement "Resistance is futile", the Borg develop into one of the greatest threats to Starfleet and the Federation. Originally perceived on screen as a homogeneous and anonymous entity, the concepts of a Borg Queen and central control are later introduced, while representatives for the Borg collective are occasionally employed to act as a go-between in more complicated plot lines.
In Star Trek, attempts to resist the Borg become one of the central themes, with many examples of successful resistance to the collective, both from existing or former drones, and assimilation targets. It is also demonstrated that it is possible to survive assimilation (most notably Jean-Luc Picard), and that drones can escape the collective (most notably Seven of Nine), and become individuals, or exist collectively without forced assimilation of others. They are notable for being a main antagonist race in more than one series who never appeared in the original Star Trek.
From Borg on Wikipedia.