Steam can be best described as a game delivery system,
It has alot going for it but does have a few niggles.
PROS: Ne need to wait for a game to be delivered, Free games and updates, A chat function and can see what games and servers your friends are playing, can compare scores with friends, no looking for Game cd,s when your wanting to play a game.
CONS: No hard copy of your games, have to be online in order to play the games, have to download every game you buy, cannot have multiple accounts for a game.
But having said all that Steam is a good way to purchase games, Iv used it for years without a problem.