I see entries in /etc/apt/sources.list that say:
deb http://us.archive.ubuntu.com/ubuntu/ hardy universe
I am using Ubuntu 8.04, and I don't know what "hardy" means, though I suspect it has something to do with version?
Hardy is the name of the 8.04 Release, "Hardy Heron".
Each Ubuntu Release follows the same pattern,
Ubuntu Releases are:
They appear on your sources list to indicate which release(version) of the repository you are using.
PS: You must have also realized by now, that Ubuntu releases follow an alphabetical naming order, always using an adjective and an animal name. The next Ubuntu release (10.10) will be named Maverick Meerkat.
PS2: The oficial Ubuntu page with all codenames, plus some discarded alternatives names can be found here, if you are curious about it.
The Letter name is the "codename" during testing--when released it becomes the number release.