Need names: online gaming companies like valve(steam) pls, here's why?

Why I need these names (since google wasn't of any help)
I know valve (STEAM), GOG and Humble Bundle (not even sure if they're all the same thing)

Now the reason: I haven't downloaded any game illegally since the time of the ancient romans (a little exaggeration here ;-) I saw an advert by steam for 'Dark Souls''. It rang a bell, youtube showed me that yes in fact I had played this game. I looked through my CDs, nothing there, Steam, nothing there, gog, nothing there. So I was wondering how on earth and where did I end up playing this game. Hence my question. I don't own xbox or anything like that but do have an account with microsoft for games when a CD required it.

In detail:

Digital distribution in video games - Wikipedia, the free encyclopedia

The current list is down towards the end.

Thanks. It was truly helpful. seeing the list showed me where to go which in turn reminded me of one not listed, 'Games for Windows Marketplace''