S2D is a clustering technology, but I list it here separate from general failover clustering because S2D is a core component of the software-defined data center (SDDC) and has had so much focus on improvements over the past few years that it really is in a category of its own.
In a nutshell, S2D is a way to build an extremely efficient and redundant centralized, network-based storage platform, entirely from Windows Servers. While serving the same general purpose (file storage) as a traditional NAS or SAN device, S2D takes an entirely different approach in that it does not require specialized hardware, nor special cables or connectivity between the nodes of the S2D cluster.
In order to build S2D, all you need is Windows Servers; the faster the better, but they could be normal, everyday servers. These servers must be connected through networking, but...