From Joomla! Documentation
Joomla! release versioning follows a numerical convention comprised of three numbers: Major, Minor and Maintenance. The version is presented in the major.minor[.maintenance] format.
- Major Release Number (X.Y.Z) - An increment of the major number generally indicates a major rework or rewrite of the code base (framework level). Can be incompatible with prior major releases.
- Minor Release Number (X.Y.Z) - An increment of the minor number usually indicates a significant change in functionality. Moderate to high level of backward compatibility with previous minor increments.
- Maintenance Release Number (X.Y.Z) - An increment of the maintenance number usually indicates bug fixing within the minor release and possibly small enhancements and limited new features. Fully backward compatible with previous maintenance increments.