All these services offer companies to host their web based applications in ‘the cloud’. For Amazon you upload a virtual machine that you can start as many instances you need of. Google chooses to only allow Python code that is then distributed over as many cores as is needed for the moment. Microsofts solution is in between. You can use any .Net language and it is up to you to configure how many cores to use. I use ‘core’ in the vaguest sense here, I don’t have a clue how virtualized the environments are.
In the usual Microsoft spirit, the tools for developers are outstanding. You can even emulate a cloud of your own in your development environment.
Sadly, Microsoft hasn’t published any pricing information yet. I think a lot of developers would have started on ‘real’ projects already if there was some indication of the pricing level. Both Amazon and Google charges for storage, traffic and CPU-usage in different way, but Google App Engine if free until it gets a lot of page views. To me, building simple services for the small Swedish speaking market Googles model is quite nice. It’s free until the site gets popular enough to support it self.
I hope that Microsoft will offer companies to host Windows Azure in their own data centers. That way one could scale out to Microsofts cloud when there is not enough capacity in house, but still keep the data that often has legal issues local.