SAN, DAS and NAS

DAS - Direct-attached storage

The main protocols used for DAS connections are ATA, SATA, eSATA,[1] SCSI, SAS, and Fibre Channel.

NAS - Network-attached_storage

NAS provides both storage and a file system

SAN - Storage Area Network  (存储区域网络)

SAN protocols include Fibre Channel, iSCSI, ATA over Ethernet (AoE) and HyperSCSI.

NAS vs. SAN

One way to loosely conceptualize the difference between a NAS and a SAN is that NAS appears to the client OS (operating system) as a file server (the client can map network drives to shares on that server) whereas a disk available through a SAN still appears to the client OS as a disk, visible in disk and volume management utilities (along with client's local disks), and available to be formatted with a file system and mounted.