WordPress.org

Codex

Interested in functions, hooks, classes, or methods? Check out the new WordPress Code Reference!

Category:API

These pages are holistic overviews of a given API or Application Programming Interface. Each covers the functions involved in and use of a given set of functionality. Together they form what might be called the WordPress API, which is the plugin/theme/add-on interface created by the entire WordPress project.

If you've read through all of these you should have a good sense of how to extend WordPress through Plugins.