Climate Progress has blogged on Plug-in Hybrids (PHEVs) many times because they are an important part of the solution, but certain questions keep coming up in comments, so here's a short FAQ.
What is a Plug-in Hybrid Electric Vehicle (PHEV)?
A PHEV is like a regular hybrid electric vehicle (HEV) but with a larger battery pack and a plug so the batteries can be charged from standard household current. Thus the vehicle is dual-fuel: your driving can be powered by either electricity, gasoline, or both. For more, see CalCars' All About Plug-In Hybrids and Wikipedia's Plug-In Hybrids. A number after the PHEV acronym indicates the electric range of the vehicle, so a PHEV-20 can operate on electricity instead of gasoline for 20 miles.