shortcut key for nearly all Windows Operating Systems to Close the active window or application.
Also used as an insulting tool by the 1337 or l33t people to have a bad/angry gamers, aka noob, to trick them to quit the game.
friend: dude, how do I quit this?
me: hold 'Alt' and press the 'F4' key at the top of the keyboard.
l33t: to fix the lag, hold Alt and press F4
<random noobs log out>
l33t: sweet, less lag...works all the time.