The Cloud OS is Microsoft's hybrid cloud solution comprised of Windows Server, Windows Azure, System Center, Windows Intune, and SQL Server. Microsoft Cloud OS vision enables one consistent platform and experience of cloud services hosted inside Microsoft Azure Public Cloud data center, in a customer's on-premises data center, or in a service provider data center. Cloud OS essentially does the same traditional infrastructure operations such as provisioning servers and applications, but with cloud scale capabilities and efficiencies.
Windows Azure Pack adds major contribution towards enabling Cloud OS vision, which brings Microsoft Azure's capabilities and experience inside an organization's private cloud and service provider cloud. This provides one true, consistent platform to deliver cloud services using Microsoft technologies.
It can be summarized in the following quote from Microsoft's Whitepaper, "Unified Management for Cloud OS: System Center 2012 R2":
"The Microsoft vision for a new era of IT provides one consistent platform for infrastructure, applications, and data: the Cloud OS. The Cloud OS spans your datacenter environments, service provider datacenters, and Windows Azure, enabling you to easily and cost-effectively cloud optimize your business."
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More information on Microsoft Cloud OS can be found at http://www.microsoft.com/en-in/server-cloud/cloud-os/.