I'm not sure I understand the question. If you mean what's the difference in the way data is accessed, I'll try and explain that.
For NAS, the remote file system on the NAS device is "mounted" on the client that will cause a redirection of file accesses to be done across the network to the NAS device. It will appear as a local file to the client but the remote access is handle "automatically" by the redirection (and the TCP, IP, device drivers and NIC adapter layers).
For a SAN, the LUNs are available to the server just as if they were direct attached disk. There may be management differences including setting up LUN masking for security controls, but the application and operating system I/O is done as if it were a locally attached disk.
Evaluator Group, Inc.
This was first published in July 2001