Fortify vs Sentry command

What is the difference between sentry and fortify with zone-of-control? Artillery, for instance-- They will fire when an enemy enters their ZOC, but what do they do if commanded to be on "sentry" duty? And... "sentry only enemy" is different than just plain "sentry"- does that mean that ALL units would be attacked? I've played this game a lot so far, and cannot find a real difference between fortify and sentry... Unless I'm doing something wrong?

Fortify means that a unit will stay in that tile and gain an extra 50% added to defence if the unit is attacked

Sentry is when a unit will become active once an oppenents unit moves next to it, enemy only is when only a civ who you are at war with will activate your unit