Beryl was an upstream fork of the original Compiz project. It was the name given to the popular Quinnstorm version of the compiz and was decided that it was to be forked after numerous changes were made in the core.
The main differences were the renaming of the window decorator to "emerald" (with extra features added), as well as the inclusion of many extra plugins which enhanced usability as well as providing extra "eye-candy".
For a time, Beryl was a more popular choice because of its rapid development schedule and the ease of use of its theming and configuration tools.
Beryl has been re-merged back with Compiz and mostly appears in the form of the extra Compiz Fusion plugins.