Joomla CMS rolls out regular updates from time to time. For this reason we have different Joomla versions. The Joomla versions include Joomla 1.5, 1.6, 1.7, 2.5 and 3.0. The purpose behind these is mainly aimed at including new features and getting rid of any bugs or errors in existing versions. Some of the important difference between Joomla versions have been highlighted below.
1. Joomla 1.5 compared to Joomla 1.6
2. Joomla 1.6 compared to Joomla 1.7
3. Joomla 1.7 compared to Joomla 2.5
4. Joomla 2.5 compared to Joomla 3.x
These are some of the basic difference between Joomla versions. For complete list of difference between Joomla version, refer official Joomla site. To empower your Joomla websites and enjoy newer features, a Joomla Upgrade is necessary.