Basically when you are driving slow and are given it less then 50%+/- throttle then it runs on the electric motors alone powered by some large expensive batteries. When you give it more then 50% (or something like that) throttle, the gas engine starts up and powers the wheels while charging the batteries.
HowStuffWorks "How Hybrid Cars Work"
I would get into why a Hybrid is a complete waste of money and barely gets any better mileage then the gas comparable vehicle while using a non-recyclable battery that cost $5000+ and requires more fossil fuels to produce then it "saves" but that is just another 40 page thread we don't need here.