Bitcoin integration/staging tree
Developers work in their own trees, then submit pull requests when they think their feature or bug fix is ready.
If it is a simple/trivial/non-controversial change, then one of the bitcoin development team members simply pulls it.
If it is a more complicated or potentially controversial change, then the patch submitter will be asked to start a discussion (if they haven't already) on the development forums: http://www.bitcoin.org/smf/index.php?board=6.0 The patch will be accepted if there is broad consensus that it is a good thing. Developers should expect to rework and resubmit patches if they don't match the project's coding conventions (see coding.txt) or are controversial.
The master branch is regularly built and tested (by who? need people willing to be quality assurance testers), and periodically pushed to the subversion repo to become the official, stable, released bitcoin.
Feature branches are created when there are major new features being worked on by several people.