
Calling a car a hybrid signifies that it's driveshaft can be turned both by an electric motor and a combustion engine. A plug-in hybrid is a car that has extended electrical capacity supplied from the grid allowing for extended driving in all-electric mode. Modified Priuses and the upcoming Plug-in Saturn VUE are examples of those.
EVs and BEVs are cars that only have an electric motor and a rechargeable battery. They usually have overall limited ranges. The Tesla and EV-1 are examples.
GM toyed with different terms to describe the Volt. They have decided on the term E-REV (with the dash, pronounced ee-rehv), which stands for extended-range electric vehicle. They like the marketing opportunities of "REV" (i.e. E-REVolution)
The graphic above is a screenshot from a lecture just given by Tony Posawatz and Denise Gray on the current state of Volt development. You will soon see the video here in it's entirety (24 minutes long).
So this nomenclature issue is now settled.