They don't. Only games distributed and managed by Steam can make use of the Steam API.
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.
It is an addon that you buy from steam to use with other games to give them more features.
Yes you can.
This is possible, but only if the game already is available to buy from the Steam store. For example, if you have Half Life 1 from back when it was not on Steam, you can validate the CD key on Steam...and then play it without the disc through that. Games that Steam doesn't support however can not be translated over to Steam. Quite a few games are on Steam though, including EA recently adding a number of their games to it.
Yes
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If you want the game download a program called Steam. With Steam you can buy games and other stuff. Steam is also legal to use because you just buy games. Anyways with steam click on STORE then search Dungeon Defenders. Buy it, then install it and your ready to go from there
no you can add other games you have on your computer open up steam click top left conner bring down a scroll bar and select add program you can add chat programs other none steam games to be in your library so you can start game right from steam and use steam while playing with friends on none steam game.
A steam plant is any where from 33% to 48% efficient. With a steam plant you use steam to run an electirc generator. Water is heated creating steam. Which is sent through the steam turbine.
Use it for the steam terminals
you use the steam battery