This was taken from their EC2 page. Very interesting!
You have complete control of your instances. You have root access to each one, and you can interact with them as you would any machine. Each instance predictably provides the equivalent of a system with a 1.7Ghz Xeon CPU, 1.75GB of RAM, 160GB of local disk, and 250Mb/s of network bandwidth.
Pay only for what you use.
$0.10 per instance-hour consumed (or part of an hour consumed).
$0.20 per GB of data transferred outside of Amazon (i.e., Internet traffic).
$0.15 per GB-Month of Amazon S3 storage used for your images (charged by Amazon S3).