To find out your Ubuntu version (or codename) use the lsb_release command which will “print distribution-specific information”:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 11.04
Release:        11.04
Codename:       natty

Or if you just want the string of the codename:

$ lsb_release -s -c
natty