This article covers various techniques and methods for representing these parameters in the URL in a clean and "friendly" manner, as well as then how to read the parameters.
One of the major reasons for using a server-side language such as PHP is for the ability to generate dynamic content. Often this will lead to single scripts that produce their content based on the input parameters (that is, the variables in the URL).
They start off with some examples of what they look like, and move right into how to use the Apache mod_rewrite functionality to take in the URL parameters and map them back to a PHP script. They also use the ForceType keyword in Apache to get the server to parse the URL string correctly. They then wrap it all up with the creation of a custom 404 page to handle the errors that might come up, and a summary of the whole project...