From Joomla! Documentation

Plugins are grouped together depending on which event they run on. A full list of plugins grouped by event type can be found here (the names of a lot of events changed between Joomla 1.5 and Joomla 2.5 see here for a full list of changes). There are also several simple tutorials on making some sample plugins running on some of these events to help running searches on extensions using both the search and smart search components: