Steam requires authentication with their servers, and that authentication is cache-able for offline access, for a period up to a month before requiring reauthentication.
It is DRM, but I don't find it to be onerous.
Individual publishers may add additional DRM measures, at their option. Those can sometimes be painful, but are clearly stated on the steam store page for each piece of software.
Additional DRM measures are usually stated, but sometimes things are omitted or notifications not removed when the additional DRM gets removed. I've bought a few games before that said they still had the DRM, but they no longer did (things like secure rom, games for windows live, etc).
I'm always paranoid and check both the Big Steam DRM List[1] and also the Steam Forums to make sure there isn't any additional DRM not listed. Especially since refunds are hard to deal with on Steam.
doesn't Steam also have DRM?