DAS:

Direct-attached storage, or DAS, is the most basic level of storage which most people are familiar with, in which storage devices are part of the host computer, as with drives, or directly connected to a single server, as with RAID arrays or tape libraries. Network workstations must therefore access the server in order to connect to the storage device. DAS is ideal for localized file sharing in environments with a single server or a few servers - for example, small businesses or departments and workgroups that do not need to share information over long distances or across an enterprise.  From an economical perspective, the initial investment in direct-attached storage is cheaper.

 

NAS:

Networked storage was developed to address the challenges inherent in a server- based infrastructure such as direct-attached storage. Network-attached storage, or NAS, is a special purpose device, comprised of both hard disks and management software, which is 100% dedicated to serving files over a network. 

 

SAN :

A storage area network (SAN) is an architecture to attach remote computer storage devices (such as disk arrays, tape libraries, and optical jukeboxes) to servers in such a way that the devices appear as locally attached to the operating system. Although the cost and complexity of SANs are dropping, they are uncommon outside larger enterprises. With their high degree of sophistication, management complexity and cost, SANs are traditionally implemented for mission-critical applications in the enterprise space. In a SAN infrastructure, storage devices such as NAS, DAS, RAID arrays or tape libraries are connected to servers using Fibre Channel. Fibre Channel is a highly reliable, gigabit interconnect technology that enables simultaneous communication among workstations, mainframes, servers, data storage systems and other peripherals. Without the distance and bandwidth limitations of SCSI, Fibre Channel is ideal for moving large volumes of data across long distances quickly and reliably.