zfs question

In my reading on zfs, I do not find a discussion on the differences between a
zfs create and a mkdir.

So I have zfs pool testpool.  The server is zfs only, so testpool is mounted
in /.
So what is the difference between 'mkdir /opt' and 'zfs create testpool/opt'?

Christopher L. Barnard
-------------------
comment your code as if the maintainer is a homicidal maniac who knows where
you live.