"Fan" is short for "fanatic," and almost nothing is more fanatical than running onto the field of play during a sporting event (sometimes naked). It's also very stupid, though, and these fans usually pay the price for that stupidity. Here is an awesome video clip compilation of fans running onto the field and getting owned. Sometimes they are taken out by security, sometimes by players, and sometimes by a batboy. But it's always hilarious. Enjoy.
Also check out: People Passing Out On Live TV: The Supercut