WordPress works with a series of ‘hooks’, parts of the PHP code that allow WordPress to dynamically create pages.
As the WordPress Codex explains:
Hooks are provided by WordPress to allow your plugin to ‘hook into’ the rest of WordPress; that is, to call functions in your plugin at specific times, and thereby set your plugin in motion. There are two kinds of hooks:
1. Actions: Actions are the hooks that the WordPress core launches at specific points during execution, or when specific events occur. Your plugin can specify that one or more of its PHP functions are executed at these points, using the Action API.
2. Filters: Filters are the hooks that WordPress launches to modify text of various types before adding it to the database or sending it to the browser screen. Your plugin can specify that one or more of its PHP functions is executed to modify specific types of text at these times, using the Filter API.
Visit the WordPress Codex sections on Plugin API/Filter Reference and Plugin API/Action Reference for more information on different hooks you can use in your WordPress theme.