From Joomla! Documentation

Plugins are grouped together depending on which event they run on. For developer reference there is a full list of plugins grouped by their event type. Please note, the names of a lot of events changed between the Joomla 1.5 and Joomla 2.5 versions. Here is a full list of the 1.5 to 2.5 plugin event name 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: