It varies wildly from game to game. It's very possible to use steam as simply a means to copy the game files onto your PC, or integrate with it so tightly it's

It varies wildly from game to game. It's very possible to use steam as simply a means to copy the game files onto your PC, or integrate with it so tightly it's 100% required to have an account in good standing (as well as one with the game publisher, or whatever), or anywhere in between (a lot of games make the integrations optional, which is the best solution IMO).