The Mapbender project uses two kinds of branches
Whenever a new version is about to be released (after the feature freeze has been announced), the trunk is copied to a new branch, the release branch. This is done in order to allow new features being merged (f.e. from the development branches) into the trunk. From the release branch, a build process automatically creates a zip file containing Mapbender. Only bug fixes are allowed to be checked in the release branch.
Development branch (sandbox)
Each developer can get her own development branch (a.k.a. sandbox) to develop new features. It is recommended to branch this sandbox off the trunk, and keep it synced with the trunk at all time (this means merging the changes of the trunk into your branch). Here's an instruction for merging.
After a new feature is stable in a development branch (it has been properly tested, documented and approved by the PSC), it has to be merged back into the trunk. Again, take a look at the instruction for merging.