I'm playing with upgrading a Ubuntu 9.04 installation and ran into the "An upgrade from 'jaunty' to 'lucid' is not supported with this tool." and I had no idea what version `lucid' was.

To find the version of the currently running Linux box use lsb_release -a

lsb_release -a

Output of lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 9.04
Release: 9.04
Codename: jaunty

So here is a nice table listing the Ubuntu versions:
(note: I don't know if this is exactly right esp. re the slug column)