MySQL is an open source database management system that uses structured query language (SQL) to access, add to and manage the content of a database. The fact that it's an open source tool means that it's a popular choice for software developers, which is why it forms the backbone of many of the most popular operating systems. Most modern content management systems (such as WordPress) use it to store data. Every change that you make in the CMS is recorded in the MySQL database, and then every time a visitor arrives on the site, the content of that database is served up to them.