What is database replication and why is it used?

Replication copies data across multiple database servers. Benefits: high availability (failover if primary fails), read scaling (distribute reads across replicas), data locality (replicas in different regions). Types: synchronous (waits for replicas, stronger consistency) vs asynchronous (faster, eventual consistency). Master-slave (writes to master only) vs master-master (writes to any). Challenges: replication lag, conflict resolution (multi-master). Most production databases use replication for availability.