PostgreSQL is robust, high-quality software, supported by automated testing for both features and concurrency. By default, the database provides strong disk-write guarantees, and the developers take the risk of data loss very seriously in everything they do. Options to trade robustness for performance exist, though they are not enabled by default.
All actions on the database are performed within transactions, protected by a transaction log that will perform automatic crash recovery in case of software failure.
Databases may optionally be created with data block checksums to help diagnose hardware faults. Multiple backup mechanisms exist, with full and detailed Point-in-time recovery (PITR), in case of the need for detailed recovery. A variety of diagnostic tools are available as well.
Database replication is supported natively. Synchronous Replication can provide greater than 5 nines (99.999 percent) availability and data protection, if properly configured and managed or even higher with appropriate redundancy.