The origins and nature of the Tau has remained a mystery so far, although I recently received this rumour that might shed a little light on things (assuming it's accurate).
The Tau are apparently highly a advanced race, even though they have only recently developed warp travel. I'm told they also come from the same sector as the Tyranids do.
The Kroot will be the mainstay of their troops. There will be 2 main kinds of Tau Trooper, one with Terminator style armour and possibly even a jump pack for greater mobility. The "civilian types", as my source put it, will have probably a 3+ save and be armed with something akin to a plasma gun. They will, however, apparently be very weak with a strength of only 2 or 3. My source mentioned that evena human is likely to best them in combat - their focus is really on guns rather than close combat.
Well, this is another one of those "far out" rumours, but then this wouldn't be a rumours page without 'em. Apparently, the Necrons are in fact kind of an advanced strike force for the Tau. If this rumour is true. Which it may well not be. Who knows, fun idea either way. :)