MariaDB is a fork of MySQL. In other words, it is an enhanced, drop-in replacement of MySQL.
A drop-in replacement means that you can substitute the standard MySQL server with the analog version of the MariaDB server and take full advantage of the improvements in the MariaDB without the need to modify your application code.
MariaDB is fast, scalable and robust. It supports more storage engines than MySQL. MariaDB also includes many plugins and tools that make it versatile for lots of use cases.
MariaDB history
Sun Microsystems acquired MySQL in 2008. Then, Oracle acquired Sun Microsystems in 2010 with MySQL included.
For some reason, Michael Monty Widenius, who is the founder of MySQL, decided to fork MySQL and founded a company called Monty Program AB. He named MariaDB after his second daughter, Maria.
To avoid any company acquisition of MariaDB like what had happened to MySQL, the MariaDB foundation was created in December 2012. The initiatives of the MariaDB Foundation are to support the continuity of the MariaDB and serve as a global contact point for open collaboration.