Short answer: Lots of money.
Long answer: SAN = Storage Area Network. Terabytes upwards, typically FDDI or similar technology. Distributed, fault tolerant storage. Expensive. Won't let you down. EMC units can call the engineer before the fault exhibits itself to you. Production systems run to hundreds of thousands of dollars.
NAS = Network Attached Storage. Hundreds of gigabytes upwards. Cheap'n'cheerful by comparison, often in user-friendly plug'n'play format. Appears on the network as, say, a Windows computer with a shared directory. Much less fault tolerant. RAID 5 if you pay for it. You're the administrator and engineer for it. Costs start in the low thousands of dollars for a couple of hundred gigabytes.
Peter
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