OMG that's the $64,000 question!
It's sooooo hard to define a chav. In general they are youths of less than perfect social background, who dress in a certain way (usually lots of brand name sports wear) have certain fetishes for 'designer' labels, especially Burberry and wear lots of large, chunky, cheap jewellery. Richer chavs may manage to buy jewellery and gear that IS expensive, but still manages to look cheap!
They also have a tendancy to name their kids (of which there are usually many of varying races and ages in each family) names which in the US, are probably more than acceptable, but in the UK we think are naff and embarrassing.
Celebrity Chavs include David and Victoria Beckham, Jordan and Peter, and pretty much any Brit who ever formed an R & B group!
Famous mockers of Chav culture who emulate them to the point of ridicule include a pop group called 'Goldy Lookin' Chain', Matt Lucas (in his Vicky Pollard guise) and Catherine Tate (as Lauren).
An excellent example of Chav life can usually be found by watching The Jeremy Kyle Show (think Jerry Springer for the UK). One of the most amusing things about Chavs is that the Chavs themselves are usually unaware that they are them.
http://www.chavscum.com/
This international site probably says it best!