MySql - Master slave replication
MySql replication is a method to maintain multiple copies of a database on different machines automatically. This arrangement is referred as master-slave replication. By using this arrangement, you can take the backup of the database, you can use one database for read and another for write, you can distribute the transaction load to all databases etc.
The arrangements vary as the requirements. There could be multiple masters and multiple slaves or you could create the tree hierarchy or array of database servers. Please read other reference books or websites for details.