Origin is much like Steam, it is made to be a one stop shop for games. That is if you have a PC. You are able to purchase games and launch them through the Origin platform. Origin is finally opening the platform up to Mac users, well only a few thousand U.S. and U.K. users to start. Right now Origin for Mac is in Alpha and is far from complete.
Right now there is no store on the Mac release and the catalog is very limited. If you are one of the Alpha users you will get PopCap Bookworm free (it should show up in your games library) and you will be able to share your feedback directly with EA. You will need to be running OSX 10.6.8 and have an Intel Core 2 Duo or better CPU to get started.