Replicating data across multiple availability zones within a region to protect against data center failures. Like storing important documents in different buildings across town.
AWS S3 Standard automatically replicates data across at least 3 availability zones to survive entire data center outages.
All four store data redundantly within a single region. AWS S3 Standard and OCI Object Storage are designed for high durability across multiple facilities in a region; Azure ZRS explicitly replicates across availability zones; GCP Regional Cloud Storage replicates across multiple zones within a region.