MariaDB is a free and open source software and a fork of MySQL server. Galera is nothing but generic synchronous multi-master replication library for transactional databases. It is used in Galera Cluster for MySQL and MariaDB.
From the tutorial page:
MariaDB Galera Cluster is an open source and free synchronous multi-master cluster for MariaDB database. It is available on Linux only and only supports the XtraDB/InnoDB storage engines. There is experimental support for MyISAM, but it is not well tested. Starting with MariaDB 10.1, the wsrep API for Galera Cluster is included by default. How to install and configure MariaDB Galera as master to master replication cluster on Ubuntu 16.04 LTS