The Vsphere Storage Appliance
Introduced in 5.0, the VSA allows you to create a small cluster (2 or 3 nodes) using local storage and turning that local storage into shared storage. The VSA is managed via the VSA Manager (in the Vsphere Client) and controlled by VSAs (1 x host). These VSAs are linux based appliance which are created automatically when the VSA cluster is created.
The above example shows a built 3 node cluster. Notice that this cluster has 3 datastores (2 if built with a two node cluster).
The VMWare VSAN
Introduced in 5.5u1, the VSAN needs a minimum of 3 esxi hosts and a maximum of 32. It has functionality built into the kernel. The VSAN uses local and SSD disks (for caching and intelligent data placement) and allows for further scalability.
The above image displays how to select devices to be used by the vsan. Each host needs to have at least one internal disk and one ssd disk.
Despite the fact that vSANs are compatible with HA, DRS, snapshots, SRM and VDP, currently vSANs are not compatible with Storage DRS, DPM, FTand Storage I/O Control.