To achieve a particular effect on a WordPress blog, you have to modify how WordPress works. Some of these modifications are made to what WordPress developers call core files,� files required by WordPress to work properly.
But modifying core files is always a bad idea. It may create a security loophole. Also, you will have lost the modification when you upgrade your WordPress installation.
Still, developers need to overwrite some of WordPress’ functionality, which is why WordPress provides the Plugin API.
Hooks are one of the main building blocks of WordPress plug-ins. Almost every plug-in uses a hook to overwrite WordPress’ core functionality.
While the WordPress website itself has its own Hooks reference, the most complete hooks database / reference is the one maintained by Adam Brown, link below:
Another excellent database is WP Seek, which contains WP functions, template tags, function sources, actions, hooks and more: