When planning a 2012 server deployment, the edition you choose should be based essentially on the following criteria.

• The roles you’re planning to deploy. That means you have to determine for what purpose you’re going to use the server for (file server, active directory, exchange, SQL …?)
• The virtualization strategy. You have to ask what kind of hardware you’re going to have. Also consider if you’re going to use Hyper-V or VMware.
• The licensing. How many users you’re going to have. Keep in mind that you have to buy Client Access Licenses (CAL) separately. Also please check if your company is involved in any Licensing agreement with Microsoft.
• The budget. Last but not least. You have to know how much money you’re going to have for the project.

After considering all these, you have to choose between the following editions. Compared with the previous Windows server editions, Microsoft has simplified the process by reducing the editions. Keep in mind that there is no more 32-bit version.

• Windows server 2012 Foundation: OEM (Original Equipment Manufacturer) only edition for small business with no virtualization rights and limited to 15 users.
• Windows server 2012 Essentials: Limited to 25 users.
• Windows server 2012 Standard: Includes the full set of Windows server features. The only difference with Datacenter is the number of virtual machine instances by license. Only 2 instances of VM are permitted.
• Windows server 2012 Datacenter: Designed for powerful servers. Unlimited VM instances are permitted.

Here is the Wikipedia link for more information:

https://en.wikipedia.org/wiki/Windows_Server_2012#cite_note-long-58