Ah, Drupal hooks. One of my favorite topics.
Did you know that hooks really are one of the main reasons I love Drupal so much? No? Well now you know.
Here is a very casual, bird's-eye view (and I hope not-too-developer-oriented) explanation of what a Drupal hook is:
A hook is a way for you to place a piece of your own custom code to be run by Drupal at a very specific time. Using hooks, you can ask Drupal to run a piece of your code precisely when a comment is viewed, or a node is edited or a search result is displayed.