Steam was created by Valve, who are the makers of the half life games, to make managing video games easier. When you buy a game, it downloads right away, and the gets put into your library. Your library is the place to view and launch all of your games. Double click, click, click, click, and you are in a game.
You also get the nice community. This is where you can find other players that play the games you play. So, instead of asking everyone at school is they place this one game you have, or asking everyone at work the same question, you go online, click, click, friends to play with.
There are other features that I won't go into, but there is one I would LOVE to share. Sales. Have you ever seen a video game store where the price tag said, "Buy Just Cause 2 for 29.99 this weekend!" or, "Play this game for the weekend FREE!"? Not in a game store, but on steam, sales happen like this ALL THE TIME. DrMadMan was recently able to play Modern Warfare 2 for a weekend for free due to a sale. The sales are also weekly, so you have to keep your eyes peeled.
Join Steam today! Click on the link to join! https://store.steampowered.com/join/