Search-engine friendly URLs. Normal Joomla! URLs look something like this:
You can optionally have URLs display to look like static HTML pages like this:
Joomla! 1.5 has built-in optionos for SEF URLs. These are enabled by changing the "SEO Settings" in the Site tab in the Global Configuration screen in the Joomla! back end. There are also third-party extensions that create SEF URLs for Joomla!.