Scott Steinberg has a handy guide for parents to commonly used gamer terms in an article on ABC News' site. If you're a gamer already you probably won't need this, but if you're not, Steinberg will take you through the basics.
Basics like the definitions of avatar, free-to-play, and gamertag. More advanced things like 1337-speak and what a sandbox game is, and even things I didn't know, like the definition of machinima.
He also has a new book (wow, really? how coincidental) that I'm going to help plug but I haven't read yet. So I have no idea if it's any good or not. It's called "The Modern Parent's Guide to Kids and Video Games", and it's actually available as a free download, so hey, check it out at: www.videogamesandkids.com