Windows Storage Server is a version of Windows Server that's licensed to OEMs for use in network-attached storage appliances. Windows Storage Server 2008 included a couple of features -- namely Single Instance Storage (file deduplication) and the Microsoft iSCSI Software Target -- that differentiated it from other editions of Windows Server 2008. But there's no such distinguishing characteristics of Windows Storage Server 2012, which has no storage features beyond those found in every other edition of Windows Server 2012. Windows Storage Server 2012 is Windows Storage Server only because it is sold exclusively through Microsoft's hardware partners with storage systems such as the HP StoreEasy 5530.
Windows Storage Server 2012 is available in Workgroup and Standard editions. The Workgroup license is limited to a single CPU socket, 32GB of RAM, six isks, and 250 concurrent SMB connections. The Standard license supports 64 CPU sockets and 2TB of RAM, and it has no restrictions on the number of disk drives or concurrent SMB connections. Standard also has a number of features -- notably fail-over clustering, data deduplication, and the ability to host Hyper-V virtual machines -- that you don't get in the Workgroup edition.