Releases are made in the form of x.y.z.
x - Distribution version>
The distribution version changes less often. These versions are not upgrade compatible with previous versions.
y - Major version
This number represents large sets of changes in the distribution, this can include package name changes, removal/addition/changing of core packages ... etc. These versions are not upgrade compatible with previous versions and may contain packages which have updated or changed configuation file layout or tags.
Major versions carry a 5 year long term support.
z - Minor version
This number represents small changes in the distribution, this would include addition/removal of non-core packages, obsoletion of packages in favour of others, large numbers of package updates ... etc. These versions are upgrade compatible with previous z releases and contain all the fixes in the previous release.
Updates in minor versions are guaranteed not to affect stability and will not contain software updates which change configuration file format or obsoletes configuration tags.
Updates come in the form of a.b. This is appended to the distribution release number x.y.z to form x.y.z-a.b.
a - Critical security updates and bug fixes
These updates are released when critical bugs or security exploits/vulnerabilities are fixed. Each update contains all the updates in previous a.y releases.
b - Minor security updates, bug fixes
These updates are released when minor bugs are fixed.