ZFS is a very "abstract" concept.
You have Pools. "Pools" are just massive sets of data.
To pools you can add "vdevs", virtual devices.
To virtual devices, you can add disks.
When adding disks, you can add them alone. You can add them mirrored. You can add them in raidz1, raidz2, raidz3.
It is designed for enterprise settings, but it is easily adapted for the home.
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A 'typical' setup:
I have 4 drives. I combine them as a ZFS raidz1 vdev. This means that any 1 drive can fail and I still have backup.
I add that vdev to pool "tank". "tank" is now a massive pool that contains all 4 of my drives in raidz1.
Out of this I can create [nearly] infinite file systems: Such as "tank/Movies" "tank/TV" "tank/Pictures", etc. Every single one of those can be created. I can create virtual devices that can be formatted as UFS, ext2, ext3. Literally anything.
Somehow I manage to fill my vdev, and [and since my first vdev is the only vdev in the pool], the entire pool.
So "tank" is entirely full. That means every "zfs" that is created from that is full. Movies, TV, Pictures, etc.........
With ZFS, I can create a new vdev. So my budget is limited, so I can only add 2 new drives. I add "drive1" and "drive2" to a new vdev in mirror. I can add this new vdev to the pool. So I will add the size of one disk to the pool (since they are mirrored).
You could also add another 2, 3, 4, 5, 100 drives in raidz1, raidz2, raidz3 to the mirror. Without doing anything, tank/Movies just got larger by the size of the drives we added to it.
ZFS also checks for bit rot. It checks the integrity of every single bit. If any byte of a copy is duplicated, ZFS will make sure that you don't needlessly duplicate files. Say I copy 1MB tank/Pictures/1.jpg to tank/Pictures/2.jpg. On normal RAIDs (and unRAID included), you will use 2MB of data. With ZFS you still only use 1MB.
Again. I'll
repeat everything I said in this post.
* Sorry if I screwed the terms up, it's been a while since I've studied them. Literally set and forget.
** Glad to see the ZFS advocates finally come out. I felt like I was a loner for a while