Because most of today’s websites are dynamic and use server-side scripting, it is pretty much expected for dedicated servers to have some form of database management, either in the form of a full database server or a set of database libraries. MySQL is one of the most widely-used relational database management systems (RDBMS). It is part of the famous LAMP stack (Linux Apache MySQL PHP). If you are offering hosting services to clients, they will likely expect a version of MySQL to be installed on your Linux or Unix server. If, however, you are using your server for your own sites, you may want to consider some alternatives.
MariaDB – After Oracle acquired Sun, the founder of MySQL, Monty Widenius, forked the software and created MariaDB, an truly free and open source version that adds some features and functionality.
PostgreSQL – Described as an object-relational database management system (ORDBMS), PostgreSQL is a serious contender to MySQL, offering several unique features and high performance.
SQLite – SQLite is a public domain released set of relational database libraries that does not require a database server, making it highly portable and easy to use, even in situations where full database servers are not possible.
Cassandra – Original developed by Facebook, Cassandra is now an open source Apache project that is a NoSQL solution, offering high scalability and many other features that make it appealing to heavy traffic and high content sites.
There are many other database options out there, and there are plenty of reasons why you might want to consider any one of them. MySQL may be one of the most popular options, but it is certainly not the only one.