Download Steam and turn it on, you should now see the Steam store, at the top of the Steam app/thing there is in big letters "LIBRARY" hover over that and click "All Games" you should now see the games you own/have purchased.
Yes you can, you import and add non-steam games to your Steam library. From within the library you can launch and run any games, although 'Steam achievements' will only be available for Steam enabled games.
They don't. Only games distributed and managed by Steam can make use of the Steam API.
Yes, some. Not all of them can be activated on Steam. See the related links for a list of the games that can be activated on Steam and for the instructions on how to activate a game on Steam.
Yes, you can import and add non-steam games to your Steam library. From within the library you can launch and run any games, although 'Steam achievements' will only be available for Steam enabled games.
No, but there is a steam function to add non steam games on to your library.
steam site
You can't.
One can get free Steam games legally through Facebook. Join Facebook today and get full version of some of the best Steam games for free. Free Steam games can also be found at MakeUseOf.
Sometimes steam does in fact have coupon codes available for their games. They also regularly have sales on their games as well.
Depends on what game? there are games that don't require specific software like steam.
Steam is the best site for PC games. Google "Steam" and download their client. It has the most reliable source of games. I have yet to find 1 glitch in a a steam game.
The best to pay for your steam games is through Paypal. Steam is a gaming platform online that allows you to not only play games but connect with other people as well.