They're just the names of the releases, Ubuntu 9.04 is Jaunty Jackalope.
8.10: Intrepid Ibex
9.04 Jaunty Jackalope
9.10 Karmic Koala
The new releases are every six months, once Ubuntu's installed, you can update to the new releases just like you do the rest of the software.
Last edited by chigurh8; 07-18-2009 at 06:58 PM.