We have been receiving a few questions on what the different capabilities and limitations are between the different editions of Windows Server 2012 R2 that are available through the OEM channel ,including Foundation, Essentials, Standard and Datacenter, so it's probably worth making sure you are away of the Windows Server 2012 R2 Products and Editions Comparison document that is available for download. When Windows Server 2012 was released, a lineup change that was introduced was the removal of Enterprise, and to help fill the gap Standard edition changed to include the advanced features of Datacenter and was licensed for two virtual instances.
A version of this document is also available for Windows Server 2012 as well, Windows Server 2012 Products and Editions Comparison.
Referring to these documents should help to clarify some of the more important differences, such as amount of memory supported, virtualisation support included etc.