The Borg are the ultimate villains in Star Trek. They are a lot like religious Christian nutcakes (both Borg and the fundies would like to assimilate everyone) as would Muslims. They are cybernetic aliens linked to a hive mind.
The Borg were first seen in Star Trek: The Next Generation. A year after they were introduced, they tried to invade the Federation. They tried again later, but the Enterprise crew pwned them both times.
Very little is known about how the Borg came to be. William Shatner wrote that they were created from V'Ger. Voyager suggested that they were organic aliens that developed cybernetic technology centuries ago.
According to the Destiny trilogy, the Borg came to be in the year 4527 BCE when the last Caeliar from the cityship Mantillis forcibly bonded with three humans from the NX-02 Columbia. In the final free thought of Chief Engineer Karl Graylock, the identity of the Borg was created. They immediately targeted the planet they were trapped on for assimilation.
Borg ships are designed for function rather than form. The classes include cube (the primary vessel), sphere (scout ship/escape pod), and diamond (command ship).
The Borg were originally going to be insectoid, tying in with a mysterious parasite seen in Season 1 of TNG. Budget problems and the 1988 Writer's Strike caused them to go with cybernetic aliens.