A WordPress theme is a collection of templates and stylesheets used to define the appearance and display of a WordPress website. Themes control the look and feel of a site, including its layout, colours, fonts, and other design elements. WordPress users can choose from a large library of free and paid themes and can customize their site's appearance by selecting a theme and then making changes to its settings, styles, and templates. Themes can be easily installed and switched, allowing users to change the look of their site without having to modify its content or functionality.