Hooks are the single most important concept to learn when it comes to module development. Hooks allow developers to tap into Drupal at virtually any point to change anything from forms to nodes to the how a bit of output displays. There are over 70 hooks in Drupal 6, but you only need to know a few to get started developing in Drupal.
In this video, I cover how to implement a hook, and show you how easy it is to expose hooks in your own module. Adding hooks to your code allows other developers to tag on functionality, just like you can do with Drupal core. Pretty neat stuff!
(Using Drupal Hooks >>)