What's the meaning of the numbering for different versions? Do they signify something?
Ubuntu follows a regular 6 month release cycle. The version numbers are formatted in Y.MM (10.04: April, 2010). Every two years there is a Long Term Support (LTS) version release which will be officially supported for two more years.
The last LTS was 12.04, the current LTS is 14.04, and the next LTS is 16.04 - the three releases between these LTS releases culminate into the next LTS, ideally viewed as "6-month baby releases that piecemeal towards the next LTS".
Typically after each non-LTS release a point release is made for the current LTS designed to maintain stability and backport changes keeping an LTS release 'supported' These are denoted as a third version number (12.04.[1-4]).
This chart and more information can be found on the Ubuntu Wiki: LTS article.
Ubuntu releases on a time based schedule. At the time Ubuntu was started the GNOME project had already established a regular release schedule every 6 months, usually September and March. So Ubuntu's schedule was "when GNOME releases, plus a little bit more to put it into Ubuntu."
The way it worked out is Ubuntu released on Aprils (hence the .04) and Octobers (.10). And it's been that way ever since, with the exception for 6.06, which had to be delayed until June due to some delays.
As it ends up releasing in the spring and fall fits nicely in the calendar and there's never been a need to shift the schedule.
Also worth noting: the first number is the year of the release.
Additionally they also have code names usually some sort of adjective and an animal name here is a list of all the Ubuntu versions including the next one.
Version Name Release Date 4.10 Warty Warthog 2004-10-20 5.04 Hoary Hedgehog 2005-04-08 5.10 Breezy Badger 2005-10-13 6.06LTS Dapper Drake 2006-06-01 6.10 Edgy Eft 2006-10-26 7.04 Feisty Fawn 2007-04-19 7.10 Gutsy Gibbon 2007-10-18 8.04LTS Hardy Heron 2008-04-24 8.10 Intrepid Ibex 2008-10-30 9.04 Jaunty Jackalope 2009-04-23 9.10 Karmic Koala 2009-10-29 10.04LTS Lucid Lynx 2010-04-29 10.10 Maverick Meerkat 2010-10-10 11.04 Natty Narwhal 2011-04-28 11.10 Oneiric Ocelot 2011-10-13 12.04LTS Precise Pangolin 2012-04-26
Apart from the points mentioned in other answers, there are few more things to add:
- Starting with Ubuntu 12.04 LTS, LTS releases will be supported for 5 years on both the desktop and the server. Previous Ubuntu LTS (Long Term Support) releases are supported for 3 years on the desktop and 5 years on the server.
- Each Ubuntu version has a fancy animal name, along with an adjective. Apart from those listed above, we have few more.
Adjective Animal Version Quantal Quetzal 12.10 Raring Ringtail 13.04