The Borg were a pseudo-race of cybernetic beings, or cyborgs, from the Delta Quadrant. No truly single individual existed within the Borg Collective (with the possible sole exception of the Borg Queen), as they were linked into a hive mind. Their ultimate goal was perfection through the forcible assimilation of diverse sentient species, technologies, and knowledge. As a result, they were among the most powerful and feared races in the galaxy, without really being a true ‘race’ at all.
The Borg were surely an inspired creation; they had an all too brief appearance in the Enterprise plotline but maybe they will show up in one of the new movies.