This is a question that comes up frequently from users. RELENG stands for Release Engineering, and is the way we label tags in our revision control software, CVS. We follow the same naming conventions as the FreeBSD project, because it makes sense and the developers are all familiar with it.
RELENG_1_2 contains the source for the 1.2 release. This branch has been frozen for more than 6 months, meaning no new features allowed.
RELENG_1 is the branch where development for the 1.3 release is currently happening. When the first 1.3 beta release nears, this will be branched to RELENG_1_3.
The 1.0 release should have been RELENG_1_0, however we did not start tagging releases until a while after 1.0 was released. This is the reason we cannot easily update 1.0 with bug fixes, and have recommended 1.2 release candidates for all deployments for several months now. This will not be the case going forward.
Lastly, the HEAD branch contains the most bleeding edge development code. It contains a lot of work in progress, and at this point we’re not sure what release that code will eventually become.