Amazon S3 is a cloud storage service that allows developers to store and retrieve any amount of data, at any time, from anywhere on the web. It provides scalable, reliable storage with interfaces for object storage and retrieval, and can be used with frameworks like Ruby on Rails through plugins and gems that integrate with S3. S3 offers inexpensive storage and data transfer rates starting at $0.15 per GB for storage and $0.10 per GB for data transfer.