They are separate versions (as can be read in the above referred wiki article).
8.04 is a Long Term Support version with three years support till (april?) 2011 for the desktop version and five years support till (april?) 2013 for the server version(by that time there should be a 10.4 with LTS).
9.04 is a more up-to-date version and will be supported for 18 month (oct 2010). The non-LTS versions are a little more a testground (my opinion)
There are advantages in both. If the LTS versions work for you,you do not have to worry about upgrading every 18 month. If you want support for the (more or less) latest hardware and possibly want the latest and the greatest, the non-LTS is the way to go; but in the latter case you probably will upgrade every 6 months.
I'll stick to the LTS versions (recently upgraded from6.06 to 8.04).
Last edited by Wim Sturkenboom; 06-13-2009 at 04:34 AM.