WordPress.org

Codex

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

WordPress APIs

The WordPress API stands for the WordPress Application Programming Interface. It can be separated into multiple API sections / topics. 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.

See also WordPress.org API.