From their site: Amazon SimpleDB is a web service for running queries on structured data in real time. This service works in close conjunction with Amazon Simple Storage Service (Amazon S3) and Amazon Elastic Compute Cloud (Amazon EC2), collectively providing the ability to store, process and query data sets in the cloud.
There's a new Amazon acronym to learn. RDS stands for Relational Database Service and it is the newest addition to Amazon's suite of web services. Unlike previous data services from Amazon such as SimpleDB, RDS is relational (our profiles for the SimpleDB API and new RDS API). In fact, it's a MySQL 5.1 instance but the main difference is that it is hosted on a virtual server instance in Amazon's data center. And it can expand and contract as needed, programmatically. Like the Amazon APIs before it, RDS was built to provide developers access to Amazon's infrastructure, with pay-as-you-go pricing based on your usage.