NAS = Network Attached Storage.
In a nutshell, it's a computer specifically designed to store data, whether it be files, video, audio or whatever. In other words, it's a file server.
I think what you're referring to, is to have two hard drives that are identical to each other. So one becomes a backup for the other. This is known as RAID 1 (mirrored).
Some NAS servers are more sophisticated than others and allow hot swapping and the like. This means if a hard drive fails, you can replace that drive without shutting the server down. The cheaper consumer level devices are not likely to offer that level of sophistication. But at the end of the day, it comes down to what level of uptime you need. For most small businesses, shutting down for 15 minutes to change a drive, is not a show stopper.
You would configure your normal computers to save and access files on the NAS server.