Network Attached Storage (NAS) provides remote file storage over a network. NAS contrasts with Direct Attached Storage (DAS) on the one hand and with Storage Area Networks (SAN) on the other. DAS is an extension of the computer itself and does not require a network. A SAN is a special purpose network, providing block-based, not file-based storage. By contrast, NAS supports regular file systems and file access, but over a data network.
Reference: http://nas-san.com/differ.html