Stable - generally release version.
Beta - this will contain urgently requested features and updates that may not have gone through thorough end-user testing. There may be unexpected behaviour. Some of these new features may affect the workings of existing features. The core set of features, such as timesheets, and documents should generally work. Not recommended for production environment, unless users accept there may be unexpected behaviours.
Alpha - this release will contain experimental and roughly tested features, enhancements and fixes. This release is intended for internal use and not recommended for production environment.