Views is one of the most important contributed modules in Drupal (right along side CCK and Image Cache) and though it is fairly easy to learn the basics of views, there are a number of advanced features that take some time to get your head around. One of these more advanced subjects is Views Argument Handling Code.
This tutorial is going to be a little too advanced for those new to drupal, but for those that are ready to get a better theoretical grasp on drupal's inner workings it may provide some useful insights.