I'm not sure if I'm reading this wrong or not, but it seems like S3 is for storage so that files can be accessed anywhere and EC2 is for computing power, to make the transactions of files etc faster. I can't seem to see anywhere about hosting websites etc. Aside from Amazon AWS, but that only allows sites that drive traffic back to Amazon.
Referenced from:
Amazon.com: Browse Amazon Web Services
EDIT: I guess what I wanted to say was that it is more like an external hard drive that's on the internet.