Most people have heard of hybrid vehicles, but are completely unaware that there are different types that use completely different technology. RideLust recently compiled The Complete Beginner's Guide To Hybrid Terminology, which explains the differences in detail. Roughly speaking, there are four types of hybrid vehicles available to the public or under development:
Parallel Hybrids use an electric motor working with a gasoline powered engine to produce motion. This is the most common type of hybrid available today, and examples include the Toyota Prius and the Honda Insight.
Serial Hybrids can be powered by either their batteries and electric motor(s), or via an internal combustion engine to power a generator. Examples include the Chevy Volt, the Fisker Karma and diesel electric locomotives
Plug-In Hybrids are parallel hybrids with additional battery capacity to boost the battery only range. A Toyota Prius plug-in-hybrid is under development for a 2012 launch.
Fuel Cell Hybrids use fuel cell technology to replace the internal combustion engine, and can be built as ether Parallel Hybrids or Serial Hybrids. Honda's FCX Clarity is an example of a Fuel Cell Hybrid.