SAN (Storage Area Network) is a term for describing a network of storage devices and servers, primarily for the purpose of backup. In a storage area network, we have the initiators and the targets. The initiators request read and write operation to the storage, the targets fulfill them. In a SAN, access is block-based, that means, the initiators ask like "give me block 1234 of lun 5". A contrast is NAS that uses file-based access ("give me file helloworld from directory /tmp").
The following table shows the different protocols and access means: