It's a program that comes with online store for you to purchase games, and allows you to keep all of your games in a single place (even if you didn't purchase it from Steam). When you purchase a game, the game is immediately put in your games list and you can play it. Steam automatically updates all games with the newest patches, keep them defragmented for best performance, and allows you to make CD/DVD/pendrive/external HDD backups, so you can install then on any computer, or you can download them again as many times as you want. You can play all of your games from any computer you want, it just needs to have Steam installed, then you log in and play. It's very efficient, and some people love it, it has plenty of features, such as: online chatting in-game (you can add friends, call them to join your server from Steam, multi-chatting with microphone, etc), internet web browser in-game, etc.
Although in its beginning (right when it was created) the program had a few problems and some people got mad about it, all of them have been fixed, yet some people still insist in believing and repeating the same old bugs as if they were still in the program, and trying to make it look like something terrible, trying to get people away from it, etc.
Haha, I love that google one. I also did get up some really interesting things when writing some of the things there. Did not allways get same results but things like (search how 2) "How 2pack died" , "How 2 cheat a drug test", "How 2 die" etc.