This article compares block storage and object storage at a high level. The examples are historical; check current service documentation before making an architectural decision.
Block Storage vs. Object Storage
Amazon S3 is object storage; Amazon EBS provides block storage volumes commonly used with Amazon EC2.
EBS block storage
Block storage exposes a volume as addressable blocks. A filesystem or application on the attached host organizes those blocks and may maintain file metadata. It is often used for workloads that need a mounted volume and random reads and writes, but suitability depends on the workload and service configuration. Block storage is not limited to EC2 in general; access depends on the product and attachment options.
Amazon EBS volumes can be attached to compatible EC2 instances and formatted or used by applications as required.
Object Storage
Object storage manages data as objects, usually consisting of data, metadata, and an identifier within a storage namespace. The service abstracts the physical storage location. It is typically accessed through an API rather than mounted as a conventional block device. Amazon S3 is an example of object storage.