In WordPress all information contained in posts, pages, categories and tags are stored in a database, as is information about settings and users. The data is organized in different parts of the database called tables, and each table has different table columns that the table can be sorted by.
For instance, when a visitor views your blog, WordPress asks the database to return information about your latest posts. The posts are stored in a table called wp_posts, and that table has a column called post_date. WordPress queries the database for ten rows in the wp_posts table sorted by the post_date column, formats those ten posts in the way specified by the active theme, and returns the result to the browser as HTML. Unless a caching solution is used, a database query similar to the one described above occurs every time you visit a page on a WordPress site.
WordPress uses a database management system called MySQL. In order to install WordPress, you need to point the WordPress installation to a MySQL database that WordPress can use to store data.