Windows Azure is the cloud services operating system that serves as the development, service hosting, and service management environment for the Azure Services Platform. Windows Azure provides you on-demand compute & storage to host, scale, and manage web applications and services on the internet in Microsoft data centers.
Details:
Windows Azur home page: http://www.microsoft.com/azure/default.mspx
Windows Azur documentation: http://msdn.microsoft.com/en-us/library/dd163896.aspx