Hybrid car technology uses a petroleum based fuel tank and an electric battery to get the best mileage. The fuel propels the vehicle and charges the battery. When the battery is fully charged it can propel the car by itself; but runs out of juice quickly at high speeds of travel. When the battery is exhausted the fuel tank takes over to propel the car and simultaneously recharges the battery.
Carbon emissions are significantly lower for hybrids than other cars because of the higher mileage that can be attained. The key performance feature of hybrids is the mileage. If you're interested in cutting carbon emissions that should be enough because you're not going to get a lot of horsepower in any hybrid car worth it's mpg rating; however you will get a lot of neat features that increase the joy of your driving experience.
For more information, read some facts about hybrid cars from Bright Hub's Mechanical Engineering channel.
