This is seriously one of the most fun school yard games to play as an adult (Red Rover and Manhunt are also really fun). A few rules Wuukiee left out:
The King serves the ball after someone gets out.
The King can make up rules (and like any good dictatorship, piss off people enough they'll all just try and kill only you).
You can't get out on a serve (keeps the King from spiking ball at people).
Usually people play that you can only hit the ball underhanded to keep people from spiking the ball.

Some other Foursquare lingo and rules:
Shoe shine: When someone hits the ball at your feet, and unless you're standing out of bounds it's an automatic out.
Inside-out, Outside-in: If the ball lands on an inside line whoever hit it last is out, but outside lines still count as your square.
Everytime you hit the ball you must say a word in a subject. If the King chooses "vegetables" then you have to say a vegetable without saying one that has been repeated before (if you can't, you're out).