Replication is nothing but maintaining another copy of table data in another database for recovery and for query (report). Replication is of 2 types:
1 Snapshot Replication
2 Multi-Master Replication
In the 1st case , new committed transactions will be copied into another database at table level. The data flows from only one direction.
In the 2nd case, data can flow from either direction. This is difficult to maintain when there is a sequence number used in the PK column.
The Multi-Master Replication is of two types - Active/Active and Active/Passive. In Active/Active replication both tables are updated/inserted/deleted. In Active/Passive, the data flows from one database to another only.