Why it matters
Databases are typically the most expensive part of a cloud footprint and the highest risk from operational mistakes. RDS reduces that risk substantially by encoding good practices as the default.
The architecture
An RDS instance is an EC2-backed database with managed storage. You choose engine, version, instance size, storage size and type (gp3, io2), and options. RDS launches the DB, applies AWS's baseline configuration, and hands you an endpoint.
Multi-AZ deployment maintains a synchronous standby in another AZ. On primary failure, RDS fails over to the standby with minimal data loss.
How it works end to end
Read replicas are async replicas in the same or different regions. Applications route reads to replicas to scale read load. Replica lag varies with write load.
Automated backups: RDS takes daily snapshots and keeps a WAL/transaction log for point-in-time recovery. Retention 1-35 days.
Aurora is AWS's cloud-native fork with 6-way replication across 3 AZs, up to 15 read replicas, and much better scalability than standard engines.