In a previous entry I wrote about a couple of default content types that Drupal creates and the difference between those two content types. In this entry I will expand on the idea of why you will want to create many more different content types of your own.
Drupal creates two default content types when you install it. These are the Page and Story content types. (Note: the default content types are renamed in Drupal 7 and are not created at all if you choose the "minimal" install profile.) The thinking behind these two content types is that a Page is designed to hold information that is long-lived. For instance, you would create your "About Us" page using the Page content type. A Story is designed to be time-sensitive information that will be short-lived. You might create a Press Release using the Story content type.