Gas Mileage

Hybrid vehicles, depending on how they're designed, can get anywhere from a 10 percent to 30 percent increase in gas mileage over nonhybrid versions of the same car or cars that are comparable in size and price.

Reduced Emissions

Hybrid vehicles have reduced emission byproducts, both because they have smaller and more efficient engines and because, during stop-and-go driving, they largely work from the electric motor. Hybrid cars have reduced particulate and other emissions as well as reduced carbon emissions.

Tax Credits

Many hybrid cars are still eligible for significant government tax credits to encourage their purchase and adoption. This can place a hybrid car in the price range of a compact or subcompact car in some parts of the country.

Regenerative Braking

One of the things hybrid cars do is capture the energy from braking the wheels through a generator system and store it back into the car's battery. These systems don't allow the car to turn for free, but they're a surprisingly large component of the overall fuel mileage.

Plug-In Hybrids

Plug-in models allow you to plug your car into a wall socket and charge it up overnight; the rough equivalent in costs per mile from running from grid electricity is 75 cents per gallon of gas once the capacity of the battery and the usual cost of electricity per kilowatt-hour are factored in.

Source:

PhysOrg.com - Pros and Cons of Hybrid Vehicles