A SAN is a Storage Area Network, for providing block-level access to storage subsystems across a network.
SANs provide block-level access to servers across a network. Commonly used SAN technologies are iSCSI and Fiber-channel (sometimes spelled Fibre-channel)
A Storage Area Network is distinctly different from Network Attached Storage (NAS). SAN systems don't provide any file-level access, although some combined (hybrid) systems exist which combine the features of a SAN device with those of a NAS.