Stable versions are for normal, every day use.
The current stable version is 6.06.00, released 2016-02-29. See the release notes for the list of new features.
Development versions are early previews of the next release.
The development version includes the latest features, but may also contain significant bugs.
Current Production Cycle Release is 6.06.01 RC4, released 2016-07-19:
Current Development Cycle Release is 6.07.00, released 2016-05-31:
Nightly autobuilds are for experimental testing only. These versions are created automatically and you may be the first human to test a particular build. Occasionally, even basic functionality can be broken. Development autobuilds are always the latest items to be introduced and can be unstable. Stable autobuilds are always for the production releases.