What is difference between Abfahrt and Ausfahrt ?
They both mean 'Exit' in the dictionary.
"Abfahrt" means a departure or a descent or downhill skiing
Abfahrt des Zuges ist um 11 Uhr.
Die Skipiste wurde für die Abfahrt vorbereitet.
"Ausfahrt" means an exit or a drive/tour
Heute Nachmittag machen wir eine Ausfahrt zur Küste
(note this usage of the word is somewhat old-fashioned. A more modern word would be "Ausflug")
On highways either can be used to mean "exit"
Wir nehmen die Abfahrt/Ausfahrt Richtung Osnabrück.
Note that since both words include -fahrt, some means of transport is always implied. You can't use either of them in the context of walking.
In the context of "fahren", "Ab-" denotes down from, "Aus-" from in to out, so both denote leaving or exiting.
Whether "Ausfahrt" or "Abfahrt" makes more sense depends on what it is used for and whether the thing you leave is considered more a confined space (leaving from "in" to "out") or a *space you drive on * (leaving from "up" to "down").
If you leave a covered parking space, that will very probably be considered an Ausfahrt, if you leave an elevated city highway via a ramp, Abfahrt makes more sense. Everything else is somewhere in-between and left to the context and choice of the speaker.
For added confusion, the official exit sign on German Autobahn shows "Ausfahrt" (As if you were leaving the Autobahn from the inside out).
For even more added confusion, Swiss German is a bit different here - Abfahrt wouldn't be used for a motorway exit.
Beyond that, both Ausfahrt and Abfahrt are used in other meanings, not at all connected to Exit.