DAK is a proprietary trigger system from Sig. It is a double action only trigger with two distinct reset points. The first reset point is about halfway through the full travel of the trigger outwards after firing. Pulling the trigger from this point results in about a 8-9lb trigger pull. If you let the trigger the full distance out, you get a lighter, probably 5-6lb trigger pull, but a longer trigger travel.
Turtle is right on target with his description.
However it doesn't describe how phenomenally smooth & consistent Sig's DAK trigger system is, I think that the best comparison I can give would be to that of a finely tuned Colt Python trigger.