You take careful careful aim with your weapons are you attempt to angle your shots so they land dead aim on your target, causing the most amount of damage possible.
Of course it's all done with bright colours, adorable character's and humorous voices. My wife and I used to enjoy playing against each other all the time.
Plus there's lots of variables in the level you have control over, from round times, weapons you start with, ammo available, etc. There's also team, sound profile, and level editors included. The easy to use level editor, with multiple environment styles to play in, makes for an infinite number of terrains to fight on.
The biggest downside I would say is if you try single player. Sure it's still fun, but it gets kinda old fast. It's best and most exciting with multi-player.
A definite bargain bin pick-up for your PC if you happen to come across it.