There’s some confusion about how WordPress organizes it’s Subversion (SVN) repository. Most SVN repositories are organized into three main directories, as is best practice — trunk, tags, branches.
Though there are varying schools of thought as to how branches and tags work, WordPress follows the following system:
/trunk is where future release development occurs. Right now, WordPress development is focused on an upcoming 3.3 release. All development for this release is going into /trunk.
/branches is where 3.3 will go once it is released (or where future “branches” of the software will be housed down the road. The directory contains a series of directories that are branches from the current release development — for example, /branches/3.0, /branches/3.1, /branches/3.2, etc. What you won’t find in branches are security (or dot) releases.
For instance, when a security vulnerability is discovered, it will be patched in /trunk for the current development branch and may be backported to the previous release branch (currently, 3.2). But until the next security release of WordPress comes out for that branch, it is still considered “development” and not “stable”.
/tags is where stable releases are archived. No development goes into tagged releases. These are final releases. You will find every release here in the form of /tags/3.2.1, /tags/3.2, /tags/3.1.4, etc. If you’re looking for the latest current stable for production, this is the place to look.
When branches achieve the next milestone (i.e. a maintenance or security or “dot” release), this is the place where the code is kept.
Hopefully this makes the WordPress repository (and maybe other projects) clear as mud.