- Standard Edition license. For up to 2 processors and two virtual machines.
- Datacenter Edition license. For up to 2 processors and unlimited virtual machines.
There are also no limitations on memory based on edition.
So, if you are going to run Windows Server 2012 directly on hardware, there is no difference between Standard and Datacenter editions. So, it makes sense to use Standard edition.
If you are going to run lots of VMs on a 4 processor server, then it probably makes sense to buy 2 Datacenter licenses (2 processors each) and be licensed for an unlimited number of VMs on that server.
There is some middle ground where it may be worth your while to run two Standard licenses on a 2 processor server for 4 VMs instead of a single Datacenter edition, but you'll need to do the math on that based on pricing from your reseller.
Microsoft has an FAQ here:
Editions for Small BusinessThere are also two editions for small business:
- Windows Server 2012 Essentials. This edition is meant to replace Small Business Server 2011. It has some of the nice automated stuff found in SBS, but no longer has Exchange Server. It is assumed that clients will use a cloud-based service such as Office 365 for email. Up to 25 users can connect to and Essentials server. No user CALs are required.
- Windows Server 2012 Foundations. This edition is available only from OEMs and is limited to 15 users. It is meant to be a basic file and print server that is very inexpensive. It does not support being a Hyper-V host.