A yob (or yobbo) is an aggressive, impolite young man (or teenager) who engages in antisocial behaviour and/or drunkenness.
The word is believed to have derived from spelling "boy" backwards (maybe because a yob is the antithesis of a good boy).
It is usually used as an insult, so calling someone a yob or a yobbo can be dangerous.
Be careful if you go to a pub on Friday or Saturday night. The yobs will be out and lots of them will be looking for a fight.
Lots of yobbos travel to places like Spain or Greece and get drunk, behave badly, and damage the image of Britain and the British.
You can read and find out more about yob culture here in the first part of a BBC series of features focusing on different aspects of crime.
Just for fun: