It's a difficult question to answer. The terms are used so interchangeably that PC can mean it is running Windows or Linux (technically I can boot all 3 from either a `Mac` or `PC`, you just need to know what you are doing). Mac is pretty much self explanatory and people who use the term `Mac` in reference to a computer are almost always referring to a machine running MacOS.
Additionally, Linux games do not require that it run Ubuntu OS. The Steam package that is available on the Steam site should run on any Linux/Gnu OS that can accept .deb packages (which is a rather large list these days, even OS's like Puppy Linux and Tiny Core can install .deb's). I personally prefer operating systems like Funtoo or Arch Linux and they have native and community provided steam packages.