agylardi: You know, when someone give a code without saying for which digital distribution, is it for steam ?
If its XXXXX-XXXXX-XXXXX, then yes, it's Steam. People usually say when it's not, but here's some other common ones:
GOG keys are 4x4 or the time-limited 1x8 keys (usually come from bundles/other resellers).
Desura can be of various types, I think, but 5x5 seems to be common.
Gamersgate keys are GAMID-XXX-XXX-XXX, though I've also seen other patterns after the initial string. Alternatively they use 10-Hex codes for giftcards.