A vehicle is a hybrid if it utilizes more than one form of onboard energy to achieve propulsion. In practice, that means a hybrid will have a traditional internal-combustion engine and a fuel tank, as well as one or more electric motors and a battery pack.
Hybrid cars are sometimes mistakenly confused with electric vehicles. Hybrids are most often gasoline-burning machines that utilize their electric bits to collect and reuse energy that normally goes to waste in standard cars. Theoretically, diesel-electric hybrids would be even more fuel-efficient, but hybrid systems and diesel engines both represent extra cost. So far, installing both in the same vehicle has proven to be prohibitively expensive.