Its mostly a U.K thing...the urban dictionary pretty much covered it I think. its funny cos I never used to call them chavs, they used to be shazzes and kevs, but now everyone calls them chavs.
I'd elaborate more, but as I say, I'm fairly sure urban dictionary covered it.
Oh and if you're interested some "celebrity" chavs are: Jordan/Katie price, Mike Skinner from "the streets" Lady Sovereign (sp?) I think shes the best example. Google them if you fancy and you'll see for yourself what you're missing out on