The Borg are a hive-minded cyborg race, feared throughout the galaxy due to their "assimilation" of other races. Implanted with technological devices to the point of looking half-machine, they are all linked together through networks an each of their motherships. Presumably, these motherships are all linked to each other as well, creating the "Collective" that suppresses all free will in each of the Borg drones (not unlike numerous religious cults or the results of late 20th Century Earth "must have Christmas toy" merchandising) .
Borg originated from somewhere in the Delta quadrant. Just how such a nightmare cyborg society came to be has been the subject of some debate. One early theory was the original Borg were created by some cruel empire as mass punishment for disobedience. Another theory was they were a desperate race's last-ditch effort at survival from some grave catastrophe. One former Borg had a different answer - they all started out as groupies for some music band.
Despite their lack of individuality, Borg are intelligent and adapt readily. All Borg have one arm replaced with an implant onto which a variety of devices can be socketted. Somewhat modified to different tasks, each drone typically carries an energy weapon and scanning equipment. Their bionic reconstruction grants them enhanced eyesight and hearing. They are not indestructible, but their adaptability allows them to adjust to virtually any form of attack. They are seldom surprised, though can be taken off guard by unconventional attacks. One attacking group of assimilated Ferengi which had become immune to phasers were distracted when someone threw a few bars of gold-pressed latinum onto the floor under them.
Borg spend most of their time searching the galaxy for new materials. They take any type of technology they can find and adapt it for their own use. One group of drones with reconnaissance orders were somewhat damaged after their scoutship crash-landed near a department store. Pressing on with the mission, they were found by locals having adapted TV remotes and can openers into their bionics.
Unfortunately, besides raw materials and machines they deem useful, they also consider beings of other races themselves a resource. Worlds that get their attention and starships that they come across they will try to capture and "assimilate" the people, robbing them of their free will, and adding cybernetics to make new drones. One former Borg described the beginning of the assimilation, "I started hearing in my mind 'I looooove you, you looooove me ... ' I tried to resist, but it was just too mind numbing."
One confirmed report showed they could replenish their numbers by cloning and accelerated growth, but they seem to prefer capture and assimilation, "HeLlLlOoOo ... We'Re SuPpOsEd To Be ThE gAlLaXy'S mOsT eViL vIlLaNs, ReMeMbEr? We HaVe A RePuTaTiOn To MaInTaIn."
Not every single Borg is a mindless drone. A "Queen" was once encountered by Captain Jean-Luc Picard of the Enterprise - D personally leading the Borg. She was not from the race the Borg originated from, but a "Species 137." She was very much an individual, haughty, overconfident, and ill-tempered at anything that got in the way of her goals and control. (If you think that's bad heaven help the galaxy if they assimilate K'Lynxyl) Picard commented, "She reminded me of someone I once dated in my Academy days." Indeed, she seemed particularly interested in Picard. By some coincidence, the humanoid woman Picard referred to, a master computer programmer, disappeared some years ago near an area later confirmed as having Borg activity. It is unknown what the connection is, if any.
Having assimilated who-knows-how-many civilizations, Borg technology surpasses that of the Federation and its neighbors, with superior engines, weapons, and shields. Their power was demonstrated at the battle of Wolf359, when one single mothership destroyed dozens of Federation warships. They have also been observed using time-travel, when the Borg "Queen" tried to take over Earth by going back to the 21st Century. Between these, they could potentially have assimilated the entire galaxy a long time ago. Instead, their pattern seems to be slow expansion, hammering at a world until it falls, with Earth being one of the few times they went out of their way to assimilate a world so far from their territory.
And just why did they want Earth so badly? One transmission provides one of the few clues:
"WeSsLeY mUsT dIe ... WeSsLeY mUsT dIe ... "
There have been no further attempts on the Earth itself by the Borg lately. Reports suggest they have been held somewhat in check by a race the Borg unimaginatively named "Species Number 8472." Still, they are an ever-present menace that can show up at any time.
The USS Freedom has had encounters with Borg, notably the "BoReD" whose cube had been disconnected from the rest of the Collective and drifted aimlessly without purpose. When they assimulated Admiral Ironbottom, his twisted will ended up dominating the Borg instead of the other way around. He led the cube on a rampage through Federation space until stopped by the crew of the Freedom in the Battle of New Cait, resulting in the first recorded instance of the liberation of an entire Borg cube's drones (the senior officers were court-martialed shortly thereafter for embarasing Starfleet by using someone else's ship).