Does I need to keep Steam running to play games?
Offline Mode allows you to play games through Steam without having an active connection to the Steam Network.
Can I close Steam when a game is downloading?
Steam will usually suspend that download while you are playing a game, however. If you exit Steam (Steam -> Exit), then it will bookmark the download and pick up again when you restart Steam. You can configure a lot of this in the Download settings inside the Steam client.
Does Steam running in background affect performance?
If your computer has plenty of RAM, Steam and Steam games won't really lag your PC. If not, Steam can make your PC run slower. Since your PC delicates more push to games, other programs can be slower if Steam or Steam gamesare running in the background.
Does Steam keep downloading while playing games?
Steam automatically pauses your downloads when a game is launched in order to prioritize the network activity for the game itself.
Why does Steam stay open when I close it?
Is it OK to leave my PC on overnight to download a game?
Is Gog better than Steam?
Is it better to sleep or shut down PC?