Oxford dictionary definition:
British informal, derogatory
"A young lower-class person typified by brash and loutish behaviour and the wearing of (real or imitation) designer clothes"
A chav is a rude young person from a poor background, who stands out negatively in the British society, by a discourteous and disrespectful behaviour. The main characteristics are that they wear gold jewellery, branded sportswear (which are fake or not) or Burberry clothes, and of course, the cap and the jogging into the socks. Their usual attitude is to stay in groups, which they call "crew", and to try to pick a quarrel with anybody. The term chav comes from the gypsy word for "child" but it is now generally understood as an acronym for "council house and violent", and has its female correspondent, namely chavette.
Chavs are sometimes considered as a source of fracture in the British society. As in an article of the guardian from May 2011, which presents chavs as recipient for public resentment. Indeed it has become for the middle-class people a derogatory term to refer to the less well-off and to complain about their improper behaviour. A confirming evidence of this phenomenon could be the website Chavtowns.co.uk, which sarcastically lists all British towns following the "chav-rate". "Chav fighting lessons" began to appear in the UK...
They also have a proper sociolect, which mainly includes pre-made sentences or phrases like "innit" "eye yer you alright!" "hoody means respect in it" for example.
Some of the problems include alcoholism, violence, drugs (dealing as well as consuming). Some sociologists argue that in those areas, young people suffer from unemployment and abandon from the government, so that they search new way to become respected. And those ways would be to be rough and violent. One of the evidence of this desire to use bad behaviour to be noticed is the attempt to get as many ASBO as possible. These are "Anti-social behaviour order", a kind of administrative rebuke. As it has not really any big judicial value, they try to collect them.
In 2005, a study took interest in the growing chav phenomenon. It depicted the chav culture as a strange subculture which, unlike its predecessors, lacked any association with a particular musical movement or political ideals. In 2011, a book Chavs: The Demonization of the Working Class considered the "chav phenomenon" as the proof of the hierarchy in the British class-system, which gave no chance to the lower classes to get out of their misery.
They are thus a group of disfavoured young people from disfavoured areas, who are recognized for their way of speaking, their way of clothing, and mainly for their behaviour. The latter is problematic, as they try to get respect from the rest of the society by being "rough", which for them means rude, violent and disrespectful of the rules. This attitude gets things worse, as they worsen the already bad opinion about them.
Clément Monhonval and Adrien Sevrin