MariaDB is in most aspects identical to MySQL.

Differences between MySQL and MariaDB can be found at:
http://kb.askmonty.org/en/mariadb-versus-mysql-features/
http://kb.askmonty.org/en/mariadb-versus-mysql-compatibility/

The MariaDB manual can be found at:
http://kb.askmonty.org/

The MySQL Reference Manual is available in various formats on
http://dev.mysql.com/doc; if you're interested in the DocBook XML
sources go to http://svn.mysql.com.