The naming convention is associated with the year and the month that it is released. 8.04 was released in 2008 in the 4th month, April. The previous release was 7.10 in October 2007.
Ubuntu also uses a name taken either from the Lion King movie or an African animal. From the June 2006 release these have been selected in alphabetical order and the current letter is H. So, the 8.04 release is also known as "Hardy Heron". Some people only use the year identifier and some people only use the name identifier and some people use both. I have seen one blogger describe that he uses "Hardy" at home and "Ubuntu 8.04" at work! Personally I prefer to use the year identifier as then I know exactly how old the version is.
You might see some releases with the letters LTS after it. Ubuntu 6.06 "Dapper Drake" LTS or 8.04 "Hardy Heron" LTS. What does that mean?
Usually the desktop and server versions are supported for 18 months. They have bug and security fixes made available for that time. However, LTS stands for Long Term Support. The desktop verion is supported for 3 years and the server version for 5 years. So far, Ubuntu have released two LTS versions, one in 2006 (in June, it was two months late, so is often referred to as 6.06) and the current version, 8.04, is also an LTS release.
To confuse matters just a little more, you might see 6.06.2 or 8.04.1. The .2 and .1 on the end are similar to a Windows Service Pack. It means that all of the updates and security fixes are applied. Sometimes you'll be able to download the CD or DVD with all of these included so you don't have to download and install them afterwards.
The next version is due for release on the 30th of October 2008. It will be 8.10 and has the animal name of "Intrepid Ibex". The release after that has only just been announced and will be named "Jaunty Jackalope" and is due in April 2009.
Out of interest, the versions as they currently stand are:
* 4.10 "Warty Warthog" (the first Ubuntu and named before they started using the alphabetical system)
* 5.04 "Hoary Hedgehog"
* 5.10 "Breezy Badger"
* 6.06 LTS "Dapper Drake" (this was the first of the alphabetically named releases and the first LTS release)
* 6.10 "Edgy Eft"
* 7.04 "Feisty Fawn"
* 7.10 "Gutsy Gibbon"
* 8.04 LTS "Hardy Heron" (the current version as at the time of writing)
* 8.10 "Intrepid Ibex" (currently in the Alpha testing phase)
* 9.04 "Jaunty Jackalope"