What is the difference?

RELEASE: Tested after integrating new features, more stable than NIGHTLY. (Odexed builds)
NIGHTLY: Built daily with newest code and experimental features, might contain undiscovered bugs. (Deodexded builds)
HISTORY: Final odexded builds once a newer Android version is out and being built.
EXPERIMENTAL: Released when a new device is added or when a new feature is added for public beta testing and feedback. (Deodexded builds)
UNOFFICIAL: Maintained separately by individual developers, usually involves modification of shared code which cannot be merged (affects other devices), therefore maintained by the developer himself.
Note: We always test the shared code for bugs, however sometimes we may not be able to detect device specific bugs. If you have such issues, it will be better to report to the device maintainer.