Since there is already a topic about this, I'm going to go ahead and merge them.
Origin is EA's version of Steam except it's for only their games. When you register a game, all of your EA games become available for download via Origin where your information is kept like the registration code. It's helpful in case something happens to your discs. It also has several options that are similar to steam including the ability to talk to friends. I have it but I rarely use it and now it's fully available for Mac users as well and they are going to start offering Mac downloads of your content through that client.