Ah cool, thank you. So, I could potentially have this little box attached to my router, which is in turn attached to my PC and have it running so that my files can be stored on it? The little OS, does that back stuff up automatically or would I need to tell my PC to send files to it? I'm guessing that there's software that would do this for me.
Also, the little box... would it need a monitor/keyboard/mouse permanently attached to it or could I access it via my current PC?
With any modern NAS you basically just connect your shiny new box of tricks to your router using a standard Ethernet cable and turn it on, it'll then grab an IP address from your router and present itself on your LAN, and you can then access it by typing in the default URL into any web browser.
Any decent NAS will essentially offer a full 'Windows style' experience in a web browser, everything is GUI driven, so can set up your shares and permissions and all that stuff in a web browser. Most also support external drives so you can also hang external drives out of the NAS (they generally have a couple of USB ports), and then access them on the network rather than directly from your PC.
I recently bought a Synology NAS which had a lot of the spadework already done, it has 2x4TB drives and it was pre-configured with a single 4TB volume using RAID-1, which means that the two drives are mirrored, so if one drive in the NAS fails, I don't lose all my data. All I had to do was set up my shares, users, and permissions.
Modern NAS boxes are generally fairly capable from a hardware perspective (featuring dual/quad core CPUs, and 512MB to 1GB of RAM), so can run all sorts of other swizzy stuff if you want, such as media servers, do hardware transcoding and whatnot.
In some regards it would be more useful to think of them as 'mini computers' or 'mini servers', rather than a dumb box with a couple of hard drives in them.
As for backing stuff up to the NAS, you can either use the built in Windows backup, a free bit of software such as VEEAM, or most NAS manufacturers offer their own backup solution along with the NAS, which can often be installed in the form of an app.
This is the fella I got earlier in the year, and I've been very impressed with it - https://www.ebuyer.com/830170-synology- ... 18j-8tb-iw