Open terminal and type:
cat /etc/issue.net
or
cat /etc/issue
Output will be like:
Ubuntu 11.10
lsb_release -a
Output will be like:
Distributor ID: Ubuntu Description: Ubuntu 11.10 Release: 11.10 Codename: oneiric
or
cat /etc/lsb-release
Output will be like:
DISTRIB_ID=Ubuntu DISTRIB_RELEASE=11.10 DISTRIB_CODENAME=oneiric DISTRIB_DESCRIPTION="Ubuntu 11.10"
uname -a Linux Tect-Computer-VM 3.0.0-15-generic #26-Ubuntu SMP Fri Jan 20 17:23:00 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux