Joomla.org announced the latest version of Joomla which is 3.0. Version 3.0.0 is considered a 'Major Release' which means it's on a different suppert cycle than long term versions like 2.5 or the future 3.5.
This release is for those that need to start using the stable version of 3.0 right away. Some use it to test extensions, or simply to build a new website on the latest Joomla platform. Joomla 2.5 will continue to be supported until a little while after Joomla 3.5 is out in the future (Spring 2014). So feel free to wait.
Support for Joomla 1.5 is ending shortly after this release of 3.0. Time to migrate to the new version!