Plugins are grouped together depending on which event they run on. A full list of events grouped by event type can be found here (the names of a lot of events changed between and 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:
There are further tutorials using the user triggers on how to create an authentication plugin to help users log into Joomla and creating a profile plugin for Joomla.
Content plugins can be used to add custom fields to core components. The following tutorial explains how to do this and how to display the new field on your webite.
A more complex example of using plugins to create a new system router to produce URLs is also available ( only).