MariaDB is based on MySQL and is available under the terms of the GPL v2 license.
It's developed by the MariaDB community with Monty Program Ab as its main steward.
MariaDB is kept up to date with the latest MySQL release from the same branch.
In most respects MariaDB will work exactly as MySQL: all commands, interfaces, libraries and APIs that exist in MySQL also exist in MariaDB. There is no need to convert databases to switch to MariaDB. MariaDB is a true drop in replacement of MySQL! Additionally, MariaDB has a lot of nice new features that you can take advantage of.
See the MariaDB FAQ for more information.
The current stable MariaDB release is MariaDB 5.2.
The previous stable release is MariaDB 5.1.
The development release is MariaDB 5.3 which is now in beta.