StingingVelvet: To me DRM is the company controlling your access to the game after the sale and download. If you can buy the game, download it and then never need authorization again to install and play then the game is DRM free. If you need the company again at any point to play, the game has DRM.
GOG is DRM free because you can download a game and never need GOG again. Steam is (usually) not DRM free because the games require Steam to run which requires getting online and authenticating when moving to a new computer or after a certain amount of offline time (supposedly).
I would say that's pretty much the gist of it.
There are some people (apparently including the people who wrote the Wikipedia article) that include traditional disc-based and manual-based copy protection methods, but the term DRM is misleading for those applications, because there isn't any "rights management" per se - all it does is hinder copying (which is coincidentally the same aim as DRM).
The term digital rights management was coined as a marketing term to specifically indicate that content providers could retain control how their content was used post-sale, unlike "local" anti-piracy measures. Part of the marketing blurb of the time specifically stated that it could be used not only to inhibit piracy, but also prevent used sales and allow "trial periods".