Origin is the new name for the EA Download Manager, a simple application for downloading and playing games from Electronic Arts.
If you prefer to buy digital copies of games instead of going out to the shops, for Electronic Arts games you need to download this application. As well as managing your premium downloads, Origin also initiates decryption of the games, runs the installer and launches it. If a game requires a DRM (digital Rights Management) license, this will be acquired by Origin too.
As well as downloading games, this app also keeps you up to date with patches and updates for your games, so you'll always be up to date. It is worth noting you cannot run downloaded games without Origin, as the DRM system requires it to be running. 2011's Battlefield 3 will be released exclusively through Origin on PC.
Origin is a perfectly capable download manager, although it is obviously limited to EA games. Time will tell if it competes with Steam or not.