Steam is a program, you register, you can buy games, create custom maps for that game with some tools, add servers in a game to favorites as long as it's online, add friends, play with friends, buy more games, get more friends, play more games, buy more games, there is VERY many people who use steam aswell and the last thing, you'll end up being a geek playing Counter-strike: Source all day.
This sentance is fail, but i always win, unless i lose.