Development builds
Development builds are regular builds of the entire system that have a large number of developing packages included, as they are being tested, broken, fixed and improved. Any given build is likely to have some new features not available anywhere else, as well as some broken old features that are not working because of work on the new.
Release candidates
When an announced release date for a new stable build approaches, say 6 weeks out, work on the development builds slows down -- or at least on a branch slated for release -- and efforts focus on producing a mostly-stable release candidate build that has few or no known major bugs and is suitable for wide testing by a cadre of community members and developers.
A build that is not sufficiently stable and has bugs that interfere with a variety of related tasks is difficult to test. On the flip side, producing a mostly-stable build means throwing out features that are too complex or not fully refined, and a build that does not have many improvements over the last stable build may not be greatly worth an extended testing effort.
A fully tested release candidate becomes a stable build.
