Windows Server 2012 comes in four editions
Windows Server 2012 Datacenter and Standard are technically identical – there is absolutely no difference between them with regard technical capabilities.
The only difference is the virtualisation rights included with the license (two VMs for Standard and as many as you like for Datacenter). So if you’re virtualising you should be using Datacentre – that’s the only decision you have to make. As a rule of thumb, once you get to around five VMs you should have gone with Datacentre.
Windows Server 2012 Essentials has not released to manufacture (RTM) yet, it is still in a Release Candidate stage. It is targeted at Small and Medium businesses with less than 25 PCs. It is an ideal first server that works seamlessly with Office 365. You can download the RC here and learn all about it here.
Windows Server 2012 Foundation is only available pre-installed onto a new server from an Original Equipment Manufacturer (OEM). It is designed for organisations with up to 15 users who need a great general purpose (and economical) server.