The general difference between SAN and NAS is that SAN is block level (In other words the device accessing the remote storage assigns a file system to the drive) and that NAS is file level (The device hosting the NAS provides a file system for the drive and the device accessing the remote device sees them through a protocol such as NFS or CIFS/SMB.) SANs will be faster, generally, due to the effects of transmitting block-level information instead of file-level information.
There are other things that are implied with the name "Storage Area Network" or SAN, but high availability isn't a requirement of them. Again, generally speaking if someone is talking about a SAN, they are referring to a drive or group of drives that are available at the block level through iSCSI, FC or FCoE, while someone speaking of a NAS will be speaking of a fileserver such as a linux box that is running NFS and Samba with the file system written by the hosting device.
SANs don't have to be block level, and NAS's don't have to be file level. We have a NAS at work that is providing both block and file level services at the same time. Because it is connected to our general LAN it cannot be called a SAN device at this time. – BeowulfNode42 – 2014-02-22T02:33:17.933
http://en.wikipedia.org/wiki/Network-attached_storage
Again, SAN = Block Level, NAS = File Level.
Note that wiki article cites no sources for those initial lines. As an example, what would you call an ethernet network that exclusively hosts storage devices providing block level access to files on the device presented as a storage LUN with iSCSI to their targets? is that network a SAN even though files are used? because the device is providing block level access via iSCSI to a LUN is it not a NAS? Things get even murkier when you introduce the notion of VLANs. – BeowulfNode42 – 2014-02-23T08:47:43.667
Look, I understand that the terms are more involved, but the reality is that a SAN is block level by consensus. You don't need a technical citation to answer the question "in layman's terms" so I find it beyond ironic that we're avoiding answering the question by giving absolutely incorrect information. If you walk into ANY DC and ask the most junior DC tech there what the difference between SAN and NAS is, he will tell you that SAN is block level, NAS is file level. So will any google search. It's sad to me that my answer is beneath one that is factually wrong. :-\ – George Spiceland – 2014-02-25T07:23:00.917