Drupal provides a lot of default template files when it comes to page, node and block output.
There are, however, a few instances when you may want to theme a page based on the content type - such as having a dedicated page for when videos are viewed (I'm doing just such a thing on this site).
What you need to do then is tell Drupal about the dedicated and specific template file you wish to use for that content type. The way to do this is with preprocess functions.
