Make Plugins Work in a Custom Wordpress Theme

2.8 Plugin API Hooks
Plugins add functionality to WordPress via "Action Hooks" (see Plugin API for more information). Most Action Hooks are within the core PHP code of WordPress, so your Theme does not have to have any special tags for them to work. But a few Action Hooks do need to be present in your Theme, in order for Plugins to display information directly in your header, footer, sidebar, or in the page body.