a NAS(network attached storage) as the name implies is a collection of drives connected over the network. a DAS (direct attached storage) is a collection of drives connected directly, usually over thunderbolt or high bandwidth connection. A typical USB drive is functionally a DAS.
Directly Attached Storage. A NAS but without the network part, a box of drives which you plug into your PC directly. For consumers via USB mostly, for larger applications via SAS or something.
It's a connection standard for enterprise hardware, think of it like your PCs SATA connections on steroids. You can have higher speeds, many drives on a single cable, and redundancy.