Relations between Linux distributions releases and updates

I can't explain the relations between updates for every release and the release itself.

Let's make an example. Ubuntu has 14.04 LTS and 14.10. Both of them are still supported, so what is the difference if I apply updates? i.e., from http://ift.tt/1x5TGel I can clearly see that weechat package is updated to version 0.4 in 14.04, while it ships (or updates to) in 14.10.

The question is: what is the purpose of OS updates in every release and how the updates are entering (or not) in a release?