Joomla 1.0 was released on September 22, 2005 as a re-branded release of Mambo 188.8.131.52 that combined other bug and moderate-level security fixes.
Joomla 1.5 was released on January 22, 2008. The latest release of this version was 1.5.26 on March 27, 2012. This version was the first to attain long term support (LTS). LTS versions are released each three major or minor releases and are supported until three months after the next LTS version is released.
Joomla 1.6 was released on January 10, 2011. This version adds a full access control list functionality plus, user-defined category hierarchy, and admin interface improvements.
Joomla 1.7 was released on July 19, 2011, six months after 1.6.0. This version adds enhanced security and improved migration tools.
Joomla 2.5 was released on January 24, 2012, six months after 1.7.0. This version is a long term support (LTS) release. Originally this release was to be 1.8.0, however the developers announced August 9 that they would rename it to fit into a new version number scheme in which every LTS release is an X.5 release. This version was the first to run on other databases besides MySQL.
Joomla 3.0 was released on September 27, 2012. Originally, it was supposed to be released in July 2012; however, the January/July release schedule was uncomfortable for volunteers, and the schedule was changed to September/March releases. On December 24, 2012, it was decided to add one more version (3.2) to the 3.x series to improve the development life cycle and extend the support of LTS versions. This will be apply on 4.x series also.