The difference between NAS or network attached storage and DAS or direct attached storage is speed of access. Network drives are governed by the speed of the network and it usually maxes out at 10 gb for the faster systems. By contrast, Thunderbolt connections can do 40 gb. On the other hand NAS drives can be accessed from multiple locations on the network but the DAS has to have a direct physical connection by cable to the computer using it.
Not that I know this stuff myself, this is what people who know this stuff tell me.
I use an 8 gb SSD drive for my active processing and a RAID DAS for backup.