zpool add - This adds a vdev to a pool. This generally results in more storage space.
zpool attach - This attaches a device to a vdev in the pool. This generally results in more redundancy.
Adding a RAID Z1 vdev to a pool
zpool add testpool raidz1 ada4 ada5 ada6
Adding a mirrored vdev to a pool:
zpool add testpool mirror ada4 ada5 ada6 zpool add otherpool ada4
NOTE: This will increase capacity.
A Disk can be attached.
zpool attach testpool ada1 ada4
NOTE: This will add ada4 to whichever vdev has ada1.
A disk can also be detached:
zpool detach testpool ada4