Microsoft Azure Stack is a totally new thing, currently in “technical preview” phase. It changes currently model of using Azure. Before it, you could use a lot of Azure features (Management Portal, VMs, Storage, Azure SDK…), but hosted in the Azure cloud within Microsoft data centers. Now, with Azure Stack, you can use all these Azure features in your own datacenter in your private cloud.
Maybe you think, it is replacement for Windows Server, but it is not a true. Azure Stack is a cloud platform and enables you to make your private cloud platform where you can run Windows Servers or some other servers. Azure Stack makes Azure on not exclusive Microsoft datacenters. I think, this will be great way of using different cloud system who enables a really good hybrid cloud. With this, we can use the same deployment and development tools to build and deploy system that resides either in the Microsoft Azure Cloud or within the some other datacenter, because we can mix IaaS and PaaS services independently of datacenter owners. Almost all features on Azure Management Portal and usage of Microsoft Azure will be the same on Azure Stack and it can make life easier for IT Pros and Developers.
If you want to learn something new, you can find Microsoft Azure Stack Technical Preview documentation here. You can also download Microsoft Azure Stack Technical Preview. Try it, but I think it’ll make a revolution in future cloud systems.