Version number :Each Ubuntu release has a version number that consists of the year and month number of the release.

Ubuntu 13.04here 13 is from 2013 and 04 from AprilVersion code name :
Ubuntu releases are also given alliterative code names, using an adjective and an animal (e.g., “Dapper Drake” and “Intrepid Ibex”). With the exception of the first three releases, code names are in consecutive alphabetical order, allowing a quick determination of which release is newer. “We might skip a few letters, and we’ll have to wrap eventually.” says Mark Shuttleworth while describing the naming scheme.

List of all versions :
sources :1.