Ok, quick unbiased explanation:
This bloke called Mark Duggan, known to have connections to gang activity, was killed by police. The official story is that he fired upon officers and was killed in the ensuing firefight. However, there was enough doubt about said story, not to mention a witness account that Duggan was pinned to the ground at the time, that a peaceful protest was organised in Tottenham, London by friends, relatives and others to demand answers. At some point, a group of younger armed protesters turned up, the police refused to strike up a diologue with protesters, and tensions slowly began to rise. The inevitable spark came when a girl was allegedly beaten by riot police that had been sent to the protest just in case. Whether or not the event happened is irrelevent at this point, but the rumour was enough to set off the more violent in the crowd. The original protesters, including the mans family, left at this point to disassociate themselves with the violence, but the rest of the protesters, including a sizeable crowd of oppurtunistic looters and vandals, started wreaking havoc. So far, the momentum of the original riots is spreading to urban areas around the country, whether due to an undercurrent of politically motivated bitterness or just an impulse to run rampant. I make no judgements myself, I just hope to explain as best I can for the non-brits/ brits who haven't kept up with the situation.