The advantages to using shortcodes are obvious. First and foremost, it allows page designs to become far more unique. It also relieves the website administrator from having to create a large list of custom fields in order to perform basic content insertion. [...] And, finally, shortcodes allow a design to come alive and be truly dynamic and interesting to the end user. Too many WordPress blogs and magazine websites have adhered to the format of a big title, a standard block of text, and comments. That no longer has to be case.

They talk about using the "functions.php" file for the custom functionality, who to use them in your posts and how to use them in the theme-specific instances. Using the "add_shortcode" you can relate these custom functions to their codes for both simple and more advanced calls (code included).

WordPress doesn't normally allow you to add PHP code to pages or posts. That's for the best: you don't want clients to discover the power of the unlink function! However, you can create custom functions which are executed when a shortcode is encountered within the post text.

There's two code snippets included - one showing a simple function call, "Hello World" style, and another version that lets you define parameters to feed into the method. There's also a simple example of how you could allow them to include CSS styling into the code example too.