MJL, there seem to be many versions as to the real meaning of Chavs. The one I think closely describes them is "Cheap & Vicious". Basically they are white, working class, usually on the dole, as well as many other benefits, poorly educated people that wear a lot of faux bling and faux burberry, nike etc clothing.
I imagine closely to what you yanks would describe as trailer trash, except chavs usually live "wiv dere mums & da's, like, ya kno wot I mean, at the end of the day?" or in council accommodation.