Watch out! This tutorial is over 8 years old. Please keep this in mind as some code snippets provided may no longer work or need modification to work on current systems.
functions.php or the theme functions file is a template used by WordPress themes. It acts like a plugin and gets automatically loaded in both admin and front-end pages of a WordPress site. Usually this file is used to define functions, classes, actions and filters to be used by other templates in the theme. It can be used to add features and extend the functionality of both the theme, and the WordPress installation.