Block storage: Raw storage blocks. Used by databases and VMs. Low latency. Examples: AWS EBS, local SSDs.
File storage: Hierarchical directories and files. Shared access. Examples: NFS, AWS EFS.
Object storage: Flat namespace with unique keys. Unlimited scale. Metadata attached. Examples: S3, GCS, Azure Blob.
For system design: Block for databases, object for user uploads and media, file for shared application data.