Network Attached Storage (NAS) and Storage Area Network (SAN) are both types of storage available to multiple computers over a network. They provide centralised storage and it also centralises backups. Both can utilise RAID.
The difference is NAS provides file-level sharing accessible over a network usually from a single device (but with multiple drives) with its own shared file system. NAS is cheaper than SAN. SAN utilises a greater network speed, often with its own dedicated switches and fiber, it does NOT use the same network to connect to computers as NAS. It provides block level storage – which means it provides grouped storage presented to servers which can then use it as a drive. This means SANs are not directly accessible by users (unlike NAS), but are utilised by servers – such as file servers and virtual servers.