There is no set pattern, though most tend to use the major.minor.bug map. Ubuntu releases a version of it's o/s every 6 months. 8.04 was the last LTS (Long Term Support) version and has since been replaced by 8.10, 9.04, 9.10 and now 10.04 is the current LTS and most recent version (which I am running at the moment actually).
Their releases are dated rather than using the norm. 8.04 came out in April, 2008 while 8.10 came out in October, 2008. Their format is more close to [years after 2000] . [month in mm format]
Really it doesn't matter what you call them, as long as you keep them. It helps organize bug fixes. Someone calls you that is using your program without a version number and you have no idea how to help them. That's why most releases will come with a new version number and a changelog to let you know exactly what they did to make it run smooth. Think back to a game you play online that flashes a changelog and it lists upgrades to their inventory in shops, or fixed a missing floor that caused you to get stuck under the map--has happened to me in an xbox360 game recently, I hooked up to xbox live and downloaded the patched version and it was fixed (was funny because I actually found where the hole was and purposely did it a few times).
I'm rambling now and have been for a while but I hope you can scrape something out of all this.