In Database Mirroring, every server is a known partner and they complement each other as Principal and Mirror. There will be only one Principal and only one Mirror at any given time.
In reality, DML operations that are performed on the Principal Server are all re-performed at the Mirror server. As we all know, the data is written into the Log Buffer before it is written into data pages. Database Mirroring sends data that is written into Principal Server's Log Buffer simultaneously to the Mirror database. All these transactions are sent in a sequential manner and as quickly as possible.
There are two different operating modes at which Database Mirroring operates — asynchronous and synchronous.