DAS (Direct-Attached Storage), The disks may be internal to the server or they may be in an array that is connected directly to the server. Either way, the storage can be accessed only through that server
NAS (Network Attached Storage) it is optimized for file sharing in an organization.
SAN (Storage Area Networks) allows more than one application server to share storage. Data is stored at a block level and can therefore be accessed by an application, not directly by clients. The physical elements of the SAN (servers, switches, storage arrays, etc.) are typically connected with Fibre-Channel – an interconnect technology that permits high-performance resource sharing. Backups can be performed centrally and can be more easily managed to avoid interrupting the applications. The primary advantages of a SAN are its scalability and flexibility. Storage can be added without disrupting the applications, and different types of storage can be added to the pool.