The competition between Amazon, Google and Microsoft around cloud computing is tough.
It is just a raw storage space (e.g. file system) added to EC2, you can use it as you prefer, for instance you can use it to allocate DBMS files. Obviously these storages are persistent, they remain usable after the termination of one EC2 instance.
The link with S3 and SimpleDB is not so clear, but for backup usage.