Patch: Developers submit a patch, sometimes with dependencies, to Gerrit. The patches are currently based off of the master branch and are merged to the master branch after they've been reviewed
Master: What gets deployed to production
Beta cluster: A test wiki that syncs the master branches of many extensions approximately every ten minutes, which is available at http://en.m.wikipedia.beta.wmflabs.org. The Release Engineering team run browser tests against the beta cluster every night