What is a hook? – 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.
Hooks are very useful in WordPress. They allow you to “hook” a custom function to an existing function, which allows you to modify WordPress’ functionality without editing core files. In their article guys from SmashingMagazine have compiled 10 extremely useful ready-to-use WordPress hooks, along with examples and coding explanations. [via SmashingMagazine]