Amazon offers instant servers with a variety of OS's to choose from (Linux, Windows, and maybe a BSD-variant Unix flavor as well). It also offers REST-based Queuing (FIFO not guaranteed last I checked), storage, and file hosting.
Rackspace offers just one or two flavors of Linux (last time i checked..)
Microsoft Azure does not offer instant servers. It offers a platform for a developer or developers to host their offerings. Think of it similar to building a "plug-in". The dev builds the plug in, Azure hosts the plug in, offers redundancy, CPU horsepower, Service Bus, etc.. It's a different approach to the cloud (closer to that of the Google App Engine than Amazons AWS or Rackspace's offerings). You can expand the CPU horsepower as needed, and there is a defined Service Level Agreement, where Google does not offer one (last time I checked, a few weeks ago).
Different tools for different purposes. It's why "the cloud" means different things to different people. To some, it's just a virtual server sitting at Amazon. To others, it's a full-fledged dynamic host.
Hope this helps.
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