If I download a Steam game, play it, and delete it/uninstall it, will it be completely gone from my computer? I want to download Amnesia demo, but once I've played it, I'm gonnna delete it to save drive space. Will everything be gone? All the files? Thanks.
Almost certainly not.
Steam tends to leave at least some things lying around, as it doesn't actually "uninstall", though it obviously depends on the game in question.
Many games also insert their own data into %APPDATA%, %USERPROFILE% or elsewhere, which are unlikely to be removed when you delete the game from Steam.
Edit to add: most of the time, these files will be insignificantly small. But if you want to be super clean, be prepared to look around for the artfacts left behind and don't trust/expect Steam to manage it.