Which Ubuntu version is installed?

If you are just interessted in the Ubuntu version then type

lsb_release -a

gives you an output like this:

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 10.04.2 LTS
Release:        10.04
Codename:       lucid

If you like to now the kernel version then type

uname -a

Linux myhostname 2.6.32-29-generic #58-Ubuntu \
 SMP Fri Feb 11 19:00:09 UTC 2011 i686 GNU/Linux