Stable versions are for normal, every day use.
The current stable version is 6.04.01, released 22nd December, 2014. 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.
The current development release is 6.05.04, released 13th September, 2015.
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.