Trek defines a hybrid as, "A bike that has a "flat" handlebar similar to a mountain bike, but with 700c wheels similar to a road bike.".
Personally, I think the "conditions of use" for my bike type in the manual is even more descriptive. The manual basically adds these additional capabilities over a road bike, "plus smooth gravel roads and improved trails with moderate grades where the tires do not lose ground contact".
From my perspective, this simply means any bike that is designed so as to integrate some aspects of a mountain bike and some aspects of a road bike. If going fast is your only priority a road bike is a better choice, and if you have intentions of taking the bike on some difficult mountain trails, then a mountain bike is a better choice.
Interestingly enough, even a typical mountain bike has some compromises, as Trek doesn't allow for jumping on any bikes except for its' most extreme models.
The rules for a normal mountain bike according to Trek:
"Conditions 1 (road bike) and 2 (hybrid) plus rough trails, small obstacles, and smooth technical areas, including areas where momentary loss of tire contact with the ground may occur. NOT jumping."