The development style is Trunk-Based, but sometimes an important development is the subject of an ephemeral branch.
The version system is not a branch system with long term support for each one. Only the trunk gets new features, security and bug fixes that are not backported. These developments are made available to the public at each release of a new version.
The versions are numbered for the history and semi-automatic update system of the data and the database (each change is applied between the installed version and the target version).
The README.txt file at the root of SVN describes how to get the source code.
The trunk branch (for developers and testers) can be broken at any (and for a long) time and damage your database.