John C's answer is not correct, though I had to sign up to find out the differences. The documentation about pricing is really a nightmare, and the calculator is also not of much help.
The Free model is different from Shared, this is a screenshot of the control panel:
The differences are:
With the free model, you do not get any IP address to point your DNS to, so you can only redirect your URL (CREF) to the free url which is [yoursite].azurewebsites.net (so the catch is free advertising for Azure via your URL).
The shared model is almost the same as free, but you get an extra feature in the control panel to configure domains. Then you can link up the DNS using an A record.
The reserved model is a reserved VirtualMachine-like model. I'm not sure if you can install SQL-Server Express on there which might make it slightly more attractive than the shared model (where you pay for the SQL Server database). I'm still investigating this. Any way, you can add instances for load balancing in this model.
I'm on the 90 day trial right now but I'm a bit disappointed with the performance. Maybe I've deployed to the wrong datacenter. It seemed like West-europe could not provide a SQL-Server/Web-server combination, so I chose North-europe. I think it's all allot more expensive than a traditional ISV, but it does provide allot more freedom and control and scalability than, for example, PLESK.
Edit: I've written an article that compares traditional "Shared", VPS, Azure and Amazon hosting. It includes links to 3 times the same website, one on Azure, Amazon's free service and one on shared hosting.