I'd echo everything Aramis says, and add some further details.
Kafer's where covered in the following original publications:
- Kafer sourcebook (Background details on Kafer society)
- Invasion sourcebook (military course of the Kafer war)
- Aurore sourcebook (a world invaded by Kafers)
- Kafer Dawn (adventure against Kafer troops on Aurore)
These are usually easy to pick up on eBay, or get everything ever published for the original game as PDFs on one CDRom from Marc Miller at Far Future Enterprises. Also available on RPGNow.com as individual PDFs, though it's comparatively expensive to buy them all individually this way.
The basic concept of Kafers is a race that is intelligent only part of the time, getting smart as a response to an immediate threat. When threatened, an adrenaline-like substance raises their intelligence until they feel safe again. When smart, they become superb tacticians, fearless, ruthless and resourceful foes. The rest of the time they are so dumb their machinery has to have protective cages as they bash equipment in blind, animal rage when they can't get it to work. Some are smart most of the time. These officers use pain and violence to make others smart enough to carry out useful work.
Politically, they are organised as a bunch of independent warlords, or Suzerains, who form shifting alliances. One of these attacked Humanity, as he thought they represented the Kafer culture's primal fear - Smart Barbarians. By attacking humanity, he hoped to get all Kafer's under his sway in a kind of great crusade. But he bit off more than his pedipalps could masticate.
That's the low down.