A WordPress theme is written primarily in PHP. Themes must adhere to a collection of defined functions and routines in order to properly integrate with the WordPress core. Themes add a unique and professional look and feel to a WordPress site without having to change the core or administrative areas.
The WordPress community hosts thousands of themes that are available to WordPress based websites for free. Premium themes that offer extended functionality and/or support from their developers are also available in a subscription or one time purchase method.