To see what version of the Macintosh operating system you have, go to the Apple Menu (at the very top left of your screen, it looks like an apple) and select "About This Mac."
Unfortunately, it does not tell you if it's Lion, or Mountain Lion, or Mavericks. It would nice if it did! It just gives a number. Why they couldn't be bothered to write it there, I have no idea. But here is the "cheat sheet" for OS X, which is OS ten.
10.5 is Leopard
10.6 is Snow Leopard
10.7 is Lion
10.8 is Mountain Lion
10.9 is Mavericks
And there you go! Keep up to date, or not. It's a personal choice.