Yes, Amazon EC2 is used jointly with Amazon S3 for instances with root devices backed by local instance storage. By using Amazon S3, developers have access to the same highly scalable, reliable, fast, inexpensive data storage infrastructure that Amazon uses to run its own global network of web sites. In order to execute systems in the Amazon EC2 environment, developers use the tools provided to load their AMIs into Amazon S3 and to move them between Amazon S3 and Amazon EC2. See How do I load and store my systems with Amazon EC2? for more information about AMIs.