WordPress themes are templates that determine the design and layout of a website powered by WordPress. They include a combination of HTML, CSS, and PHP code that control the way the site looks and functions. Themes can be easily installed and switched out to change the appearance and functionality of a website without requiring any coding knowledge. They often include options for customizing colors, fonts, and other design elements, as well as providing support for various features such as menus, widgets, and page templates. Many WordPress themes are available for free, while others can be purchased or downloaded from various marketplaces.