What it is
vSAN aggregates the disks attached locally to ESXi hosts into a single datastore. As software-defined storage it builds highly available storage for a vSphere cluster, managed from the vSphere client.
Architecturally that is hyperconverged: compute, networking and storage in the same x86 servers rather than in separate tiers.
Why it matters
Choosing storage is tedious, and much of the effort is not technical but the seam between two areas of responsibility. With vSAN the storage sits where the virtualization already sits, which is one console fewer and one alignment meeting fewer.
vSAN is also the intended storage for VMware Cloud Foundation, which largely settles the question inside a VCF estate.
What to watch
Hyperconvergence couples compute and capacity. Need only storage and you buy hosts with it; need only compute and you buy disks with it. Where that ratio is permanently lopsided a separate tier is cheaper, which is exactly what approaches like dHCI are for.
