Understanding Drivepooling Options for Linux

As an absolute Linux user, I am still getting to grips with the way Linux works and the options available.

I am currently experimenting with pooling my hard drives, to be used as a single hard drives - I am only interested in pooling them in the way that whole files are sent to either disk, whereby if a single disk fails, the files on the other hard drives remain intact (I think this is referred to as JBOD as opposed to Raid).

From what I have read, there appear to be at least four methods of bunching drives together in this way - mergerfs, aufs, mhddfs and untionffs.

Could someone possibly help me understand what the difference between them is?

Are these programs for grouping the harddrives or is it a filesystem for doing so - I note that they all end in "fs", which presumably stands for file system?

Thanks.

Thank you - that makes much more sense now.

So the four I mentioned above are all examples of unionfilesystems?

The thing that threw me was that unionfs is very similar in name to unionfilesystem plugin.