UNIX shells

what is a shell?
tis the thing that displays the $ or % prompt

what's zsh?
tis the thing that displays zsh%
occasionally makes your head feel like it's been wrapped around a brick
originally based on ksh; now has features from ksh/bash/tcsh
can emulate sh/ksh, but why would you do that?
sh emulation nowhere near as accurate as bash
has all your normal modern shell stuff
functions, conditional expressions, job control, directory stacks, etc