Football Hooligans are a large collection of criminals/thugs who have become associated with European football (sometimes called Soccer in America) - the term has been around for several decades and the phenomena is a deeply-rooted part of the sport, though many clubs have started to tackle the culture as it promotes violent crime and rioting.
Football Hooligans merge the act of supporting one's "team" with hooliganism, the practice of engaging in mass fights, lawless riots and general disruption of order - most Hooligans wait until after a match to engage in mass brawls but some will attack and ruin games, throwing items at players, waving hate-symbols and inciting others to violence.
Football Hooligans are notorious and greatly focused on in the media, though some criticize this (much as they do coverage of anarchists) as giving thugs "free publicity".
Infamous Football Hooligans include the Hibs Casuals and the Chelsea football gangs, who developed the torture known as the "Chelsea Smile" (a variant of the Glasgow Smile) - despite their infamy in Britain these hooligans are considered "tame" compared to some hooligans from other parts of Europe, who have invented many ways to cause havoc.
Football Hooligans are associated with the stereotypical "thug" culture and are sometimes referred to as "Yobs" (though the term "Yob" can also be applied to roaming gangs of thugs, such as those who engage in "Happy Slapping" style attacks)