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.

• Dashboard Widgets API
• Database API  HTTP API
• File Header API
• Filesystem API
• Metadata API
• Options API
• Plugin API
• Quicktags API
• Rewrite API
• Settings API
• Shortcode API
• Theme Modification API
• Theme Cust API
• Transients API
• Widgets API