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The editions of Server 2012 fall into three groups.
First there are the main editions, Standard and Datacenter. These are what most businesses will license. Next there are the low-end editions aimed at small businesses, Essentials and Foundation. Finally, there are specialist editions for specific roles. These are Storage Server and Hyper-V Server. Note that both Standard and Datacenter are feature-complete. The other editions do not have any exclusive features, unless you count the simplified management tools in Essentials.
Windows Server 2012 Datacenter is designed for highly virtualised private cloud environments.
Windows Server 2012 Standard is designed for physical or minimally virtualised environments.
Windows Server 2012 Essentials is ideal for small business that have as many as 25 users and 50 devices.
Windows Server 2012 Foundation provides a Windows Server experience for as many as 15 users.
Understanding Standard and Datacenter
Windows Server Standard and Datacenter have the same features without limitation, the difference between these two editions in in the product usage rights.
The first point to understand is that Standard and Datacenter require both server and client access licensing. The server licence covers the server installation, while CALs (Client Access Licences) are needed for each user or device which accesses the server.
There is no difference in the CALs for Standard and Datacenter so that will not affect your decision. That said, there are a couple of features in Server 2012 which require additional CALs (called Additive CALs) if you use them. These are Remote Desktop Services and Active Directory Rights Management. You can think of these as premium services that are included in the product but which require additional licensing.
Note that using Remote Desktop for server administration is not a premium service and does not require an additive CAL.
The second key point is that Standard and Datacenter are licensed per processor. A "processor" in this context is a physical CPU, irrespective of how many cores it has. Both Standard and Datacenter editions cover up to two processors, if you have more processors, you apply further licences, so for example a four processor box would require two Standard or two Datacenter licences.
The only difference between Standard and Datacenter (aside from the price) is in the licensing for the guest operating system in virtual machines. If you have Standard, you can install two virtual instances of Server 2012 as well as the host, for each licence you install. If you have Datacenter, you can install unlimited virtual instances of Server 2012 in VMs on the host server.
This means you can get out a calculator (or more likely run up Excel) and work out which edition is best value for you, based on the number of virtual instances you expect to run or evaluate for yourself Note that this only applies to virtual instances of Windows Server. If you want to run many Linux virtual machines, for example, running Standard is no limitation.
If you are not using virtualising and running all workloads on standalone machine then Standard will be suitable.
Server Foundation and Essentials
Windows Server 2012 Foundation is a special edition designed to be bundled with hardware. It is an inexpensive first server solution.
Server Foundation has several significant limitations.
Server Essentials is great value for businesses with fewer than 25 users though. The fact that it is cheaper than Standard and does not require CALs is a significant saving. Should you grow beyond the 25 users, Windows Server Essentials has a simple licence upgrade option.