A nasty English stereotype.
Chavs are aggressive lower-class Brits found in the United Kingdom. A chav/ette typically wears tracksuits and trainers from sports labels like Adidas and Nike, often with a burberry baseball cap on their head and lots of fake gold jewelry. They are notorious for forming rowdy mobs and drinking in public, which usually leads to anti-social behaviour. Girlfriends are referred to as bints.
OTI, chavs are most commonly found on Bebo, Facebook and Omegle. Their online personality reflects their IRL tendencies toward obnoxious behaviour, with many of them being Internet tough guys. They will often type in an irritating manner, such as AlTeRnAtE cApS, or by changing words to more closely resemble real-life chav talk, which is incoherent enough as it is. They can be made very mad indeed by trolls, often provoking spectacular lulz.
Originating from the county of Kent, chavs have since spread out to infect the rest of the United Kingdom. The word "chav" is often said to have its origins as an acronym for 'council-housed and violent', but this isn't the case. According to Wikipedia, the word has its origins in the Romani word 'chavi', meaning child, or 'chavvy', meaning youth.
The chav is unfortunately rapidly replacing the traditional 'bowler-and-brolly' stereotype that everyone secretly wished was true.