Steam is an Online Gaming system which holds all sorts of games with many amount of features. Get friends, chat online with micro phones, play against them and more! Steam can be installed on the web or it can be installed by any game which they sell.
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If you bought the game from steam, no. and if your trying to put a game on your computer and it keeps adding to steam, than un-install steam while your trying to install the game.
it depends if it is a game that is on steam (legit) it will go in there automatically if it is another game (for example, minecraft) then in the library, find add a game and pick "add a non steam game". if its not in the list then click browse and find it yourself.
You can't share a game between two accounts on steam, you'd have to buy the game again on the other account.
It is as are all the DLC for the game
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.