Synchronous Replication is a technique used to back up and replicate data in the most precise manner. The technique calls for a back up machine to be linked to the machine holding the data and replicating it synchronously.
This means, that if you have a dynamic changing and growing database and you want to back it up using Synchronous Replication, the backup machine will start a process which “freezes” your database (so no changes are made to it until the backup is complete).
The process is accurate and ensures that the backup is the most recent one if not identical to the seconds before the process. However, there is a downside to Synchronous Replication and that is the response time and possible delays in communication. These deem the process impractical unless the backup machine and the database server are located in physical proximity.