As said above, hybrid cars use the saved electrical energy in order to move the car. The small gas powered engines act as both a charger for the battery packs of the hybrid, and also as a back-up power source for the car when the battery runs low.
I know that the Toyota Hybrids actually use the inertia of the stopping power of their vehicles in order to charge the batteries.
Honestly, I do feel that hybrids most definitely save the World gas, but they are usually over priced, and the people buying them "feel" as if they are saving money in the long run, but actually are paying an average of $4000 extra for the hybrid technology. More than likely someone with a Hybrid car would have to drive an average of more than likely 50k-100k miles in order to make up the cost they paid to buy a hybrid.
For the individual, it purely isn't worth it, but for a nation it is. Then again, looking at those whom are in power, the idea of "alternative fuels" is so very far fetched!