Joomla! is developed using a fixed release cycle. Every six months, the Joomla! Project releases a new minor or major version of Joomla!. (Joomla!'s version format is [major].[minor].[maintenance].)
Each version of Joomla! is supported by the Joomla! Project for a limited amount of time. There are two classifications for suppport: standard support (STS) and long term support (LTS). Every third release will be assigned long term support.
Please note that both STS and LTS releases, once they have reached the GA (General Availability) status, are fully functional and ready to be used in production environment. The disadvantage of using STS releases is that you will need to update your installation of Joomla! every six months, while the advantage is that you will have access to new features and extensions earlier.
For more information, see the official Development Strategy.