What is the difference, actually ? I never saw any, I must admit.
Steamworks is the programming interface to steam, it offers everything for multiplayer support like making connections and voice communication. It also offers functionality for player identification and anti cheating. And it offers a range of other things, one of them being DRM. I do not think there is a requirement to use DRM though. It offers almost everything that is needed for allowing players on the Internet to play with eachother. Something that has been tested thoroughly and does not need to be developed anymore.
