Linux: uname -a tells you what version of Linux you're running

$ uname -a
Linux rhel54-64 2.6.18-164.el5 #1 SMP Tue Aug 18 15:51:48 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux

There are other options to get selected info. You can do a "man uname" to see them.