steam by itself is a DRM. It controls that you are authorized to launch the game, etc.
However, I find this kind of DRM acceptable, since it does get in the way of the legit buyer extremely rarely.
>It controls that you are authorized to launch the game
Steam provides an infrastructure and API to perform this check, but the developers decide whether to use it. There are many games on Steam which don't use this DRM, and can be launched directly from their install directories even if Steam isn't running.
I was about to agree that Steam is DRM, but your comment reminded me of the time I bought a game (Dungeons of Dredmor?) that had a Linux port but which wasn't on Steam. I contacted the developers about getting the Linux port and they said "oh, it's in your Windows Steam game directory, just launch the executable for Linux", and it did, indeed, work fine.
That said, Steam is still not as un-DRMed as GOG, which provides a pretty installer you can copy and back up and do whatever you want with, but it's pretty good.
Only if the developer uses the Steamworks API. If you purchase a game on Steam that does not use any Steamworks, once it's downloaded, you can run it from your computer without Steam running. Many old games you can buy on steam are just the original game with a DosBox launcher that just sits in your SteamApps folder unprotected.
