WordPress is a free, open-source content management system (CMS) that can be used to build blogs or full-fledged websites. It exists in two versions: WordPress.org, which allows fully customizing and hosting a site yourself, and WordPress.com, which provides free hosted blogs with limited customization options. WordPress themes control the visual design and layout of a site, and can be switched out independently of the content. This separation of content and presentation makes WordPress very flexible and powerful.