WordPress is an open source content management system (CMS) based on PHP and MySQL that is usually used with the MySQL or MariaDB database servers but can also use the SQLite database engine. Features include a plugin architecture and a template system, referred to inside WordPress as Themes. Wikipedia
In the early days of WordPress it was used primarily for blogs (lists of posts or articles that expressed information, and later images and media) but because it was free and easy to use, users soon pushed it into creating complex websites. WordPress is still free to use, but is created by a network of programmers and coders that would appreciate support donations. The site WordPress.org offers plugins, themes, and education, and is the site for WordPress Foundation. The site WordPress.com is not directly related to the .org site, and offers accounts to host blogs created in WordPress. WordPress can also be installed in a standalone web hosting account, making the blog “self-hosted” or controlled by the owner of the blog or site. Web hosting account are usually purchased from web hosting companies, either as a shared hosting space, or dedicated web hosting space.
A WordPress site uses Themes (pre-programmed page layout, CSS, and image formats), and is organized in a group of Pages, which display Posts. Plugins are additional program code programmed by a company or independent programmer or developer that provides function to the site—e.g. forms for visitors to sign-up for a mailing list, or an interface to the payment processor. Some plugins are for the backend of the site where security, site management and administration occurs.