Open terminal and enter:
cat /etc/*release
it outputs:
DISTRIB_ID=Ubuntu DISTRIB_RELEASE=12.04 DISTRIB_CODENAME=precise DISTRIB_DESCRIPTION="Ubuntu 12.04 LTS"
Also, we can enter:
uname -a
Then we get:
Linux johnny-Rev-1-0 3.2.0-23-generic-pae #36-Ubuntu SMP Tue Apr 10 22:19:09 UTC 2012 i686 i686 i386 GNU/Linux