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ZFS (an advanced filesystem) is what most focal fossa users format / with. ZFS user have to forget everything they know about hard disks and start anew. Here is what they need to learn:

     Dataset         A generic name for the following ZFS components:     clones,    file systems,   snapshots, and volumes.
     File System     A ZFS  dataset  of type                                         files ystem     that is mounted within the standard system namespace and behaves like other file systems.
     Clone           A                                                               file system     whose initial contents are identical to the contents of a snapshot.
     Snapshot        A read-only copy of a                                           file system     or            volume   at a given point in time.
     ZFS             A local                                                         file system     and logical   volume   manager created by Sun Microsystems Inc. for highly scalable storage.
     ZFS volume      Is a   dataset  that represents a block device.                                          ZFS volumes are identified as devices in the /dev/zvol/{dsk,rdsk}/pool directory.     
     Pool            A logical group of devices describing the layout and physical characteristics of the available storage.
     Mirror          A virtual device that stores identical copies of data  on two or more disks.  If any disk in a mirror fails, any other disk in that mirror can provide the same data.
     RAID-Z          A virtual device that stores data and parity           on multiple    disks.
     A VDEV is a meta-device that can represent one or more devices. ZFS supports 7 different types of VDEV:
         File                - a pre-allocated file
         Physical Drive      - HDD, SDD, PCIe NVME, etc.
         Mirror              - a standard RAID1 mirror
         ZFS software          raidz1, raidz2, raidz3 'distributed' parity based RAID
         Hot Spare           - hot spare for ZFS software raid.
         Cache               - a device for level 2 adaptive read cache (ZFS L2ARC)
         Log                 - ZFS Intent Log (ZFS ZIL)