Last year Western Digital launch the Red series of hard disks intended for use in NAS devices. Seagate also has such a series now called the Seagate NAS HDDs. We tested the 4TB model and compared it to Western Digital's disks.
It's more than a marketing gimmick, special NAS hard drives. They have different software features and go through more rigid certification and compatibility tests. Seagate basically does the same with its NAS disks as Western Digital does for its Red disks.