Network Attached Storage - i.e. sharing files and folders, not storage blocks… Most are “sharebased” - you share a folder over SMB (Windows) or NFS (UNIX) - some systems let you share the same folder over both SMB and NFS (NetApp, Oracle ZFS Filer, TrueNAS). There are other sharing techs too - but Apple have mostly dropped AFS and default to SMB, and there’s other things like WebDAV…
I VASTLY prefer NFS over SMB…
That’s not quite “strict” enough… SAN is also “storage device accessible via a network” - “Storage Area Network” - in most cases SAN storage implements some sort of proprietary network over fibre optic - there will be a SAN switch the SAN is patched into (usually fibre) - and client machines will have a fibre HBA (host bus adaptor) connected to the switch - and SAN administrators will setup the “zoning” on the SAN switch so that Host A and Host B don’t try to get to the same allocated block storage on the storage backend of the SAN…
Some NAS will also do iSCSI - that’s a sort of hybrid of NAS and SAN - i.e. you can share a predefined “file” as a chunk of block storage - that an iSCSI client will see as a chunk of block storage (e.g. /dev/sdg, /dev/sdh etc) - and you can just use TCP/IP networking infrastructure… By and large - dedicated SAN over Fibre will be more performant that iSCSI over TCP/IP (even TCP/IP over fibre).