MariaDB Server is a world-wide popular database server, which is created by the original developers of MySQL. It is highly compatible with applications developed for MySQL. New features from MySQL are likely to be also added into MariaDB. In most cases, users can directly switch from MySQL to MariaDB without extra work.
MariaDB was forked from MySQL after Sun's acquisition by Oracle. The project is lead by Michael Widenius, who was also a founder of MySQL AB which was later acquired by Sun. The DBMS was named after Michael Widenius' younger daughter Maria, just like MySQL, which was named after his first daughter "My".