What in the hay are lag screws?

I have never heard of lag screws before. I mean I've heard of normal screws and hand screws and whatnot but lag screws? I keep having the mental image of a screw being hammed in really reaaaaally slowly. But (as I was helping a friend fix up their backyard deck project) lag screws are like other screws but one thing - they hold heavy loads. Heavy loads such as the foundation of a house or a tall hi-rise is what these types specialize in. In this deck's case, it will get lots of foot traffic and so a simply metal or wood screw wont do. The more you know!