The Borg are a cybernetic race (fictional, from StarTrek - first seen in StarTrekTheNextGeneration). They use technology to link their conciousness. So they have one conciousness - one mind. They don't kill or enslave or imprison people - they just "assimilate" them into their collective - and force their mind to be linked to the collective mind. The Borg is more sinister than any other race because there is no death with honor or dignity with them - there is only assimilation, sometimes thought to be a fate worse than death.

A good initial description of them can be found at: http://www.startrek.com/startrek/view/library/aliens/article/70558.html