There are 3 upgrade options that exist.
1. Windows Server 2012 Datacenter edition and Windows Server 2012 Standard edition.
2. Windows Server 2012 Essentials.
3. Windows Server 2012 Foundation.
Windows Server 2012 Datacenter edition and Windows Server 2012 Standard edition: There is no much difference between these two in terms of performance apart from the number of virtual machines you one run without the need of paying for more windows licenses. Datacenter edition is meant for virtualizations while standard edition is meant for application hosting.
Windows Server 2012 Essentials: Meant for small organizations and includes all the features of Datacenter and Standard editions.
Windows Server 2012 Foundation: Has no virtualization right and provides a general purpose server experience.
