MariaDB is an alternative MySQL DBMS being developed by MySQL author Michael "Monty" Widenius. The main goal of MariaDB is to create a fully binary compatible version of the original MySQL DBMS, but with a significant number of performance improvements in the code. MariaDB is developed as a drop-in replacement for MySQL, fully mimicking the behavior of MySQL.
MariaDB is a server-side program for storing and retrieving data using SQL queries. A similar class of software is called a RDBMS (relational database management system). MariaDB is an offshoot of MySQL, another management system previously created by Sun Microsystems.
In general, MariaDB has evolved faster and has more features. These features relate to optimization, improved memory handling, and much more. Usually, over time, these features are carried over to MySQL. For example, the same GIS support appeared in MariaDB earlier than in MySQL. Among other things, MariaDB has many performance improvements to Inodb, MyISAM and the query engine, support for GIS, table elimination, virtual and dynamic columns, multi-source replication, roles and more.
But MariaDB does not support some features that MySQL has. Namely, MariaDB is incompatible with MySQL JSON syntax, the ngram, MeCab, MySQL X plugins, and table spaces that allow you to assign data to multiple tables simultaneously are not supported. But the developers are actively working on fixing the shortcomings.