Ultra's, obsessed fan groups of a football clubs. The people that make giant banners (called tifo's), let off flairs, spend more time banging drums and chanting than watching the game, that kind of stuff. They usually inhabit a specific section of a stadium for every game. Has historically been more associated with football in places like Italy, Spain, South America and Eastern Europe.

It started being a bigger deal here from around the late 2000s. To the point that almost every football team in Scotland at whatever level will have some group of wee guys in ski masks and parkas in front of a banner saying "Block Eight Boys" or something. I would imagine it's probably similar in the lower levels of English football. Doesn't tend to be as big a thing in the EPL which is a bit more corporate.

As to whether they are inherently violent that's tricky. They aren't quite analogous to football casuals, people who deliberately meet up with other teams "firms" to have a ruck. Something that was huge in the 80s and had a bit of a resurgence in the 00s. But there is obviously a lot of overlap in culture. It being football they do have an association with trouble and controversy anyway.