Replication

 Replication = Redundancy + Synchronization

It involves redundancy , but involves copying of data from 1 node to another or synchronization of states between nodes.

Active Redundancy

Data in all the  server are same & in synchronization.

Passive Redundancy

Every Read-Write => Done By Master

If master goes down ,then one of the slave becomes master

Master-Slave Replication can be either synchronous/ asynchronous 

Difference is simply timing of propagation of changes.

If changes are made to master & slave at the same time , it is synchronous

If changes are queued up  & written later , it is asynchronous