It varies a bit from game to game, but typically Steam games are intertwined with Steam in one way or another. You can move the files around, but you need Steam to verify and "fix" the files and their associations afterwards.

It definitely does vary though. For example, with KSP I was able to just copy the install directory and have many different install folders for different instances of the game (great for version and mod control). For others, I was able to copy the files, but it didn't run, not until I manually set up Steam to the install directory and did the verify integrity thing.

There is a distinct difference between copying the game folder to another drive, and a steam backup.