S3 is the default object store for designs that need to store images, videos, documents, backups, logs, or data lake files.

The API is small: PUT an object, GET an object, list objects by prefix, apply policies. The design decisions behind that API are what matter in interviews.

S3 gives you high durability, elastic capacity, strong object-level consistency, lifecycle-based cost controls, and direct client upload patterns. The trade-off is that it is not a low-latency database or a POSIX file system.

In an interview, the goal is to explain exactly what role S3 plays, which guarantees you rely on, and where you still need a database, CDN, queue, or coordination layer.

The diagram shows how a PUT or GET request flows from a client through S3's API layer, how a flat object key resolves through the metadata layer to a storage location, and how every object is stored redundantly across multiple availability zones inside a region.