Both verbs dejar and terminar mean to stop certain activity. But while terminar (= to finish) supposes that one completes this activity, dejar means to abort or abandon it:
He terminado los estudios = I finished my studies
He dejado los estudios = I abandoned/left my studies
In many contexts dejar is a synonym of abandonar:
Mi padre huyó con otra mujer. Nos dejó cuando yo tenía tres años = My father left us when I was three years old
¡Déjame en paz! = Leave me alone!
The verb parar means finish a movement. In this sense it is synonymous with detenerse (more formal and elegant):
El coche ha parado en la esquina = The car has stopped at the corner
Juan se detuvo delante del escaparate de una tienda = Juan stopped in front of a shop window
However, in the meaning of "to make a stop for passengers to get on and off" only the verb parar is used:
Los autobuses no paran aquí = There is no bus stop here
Los autobuses no se detienen aquí = Buses do not slow down when passing through here
Also parar means to make a break:
Estoy cansado de estudiar, necesito parar un ratito = I am tired of studying, I need to make a break
Also we can parar or detener something or someone. Although they are synonyms, detener sounds more elegant:
Tenemos que parar la violencia / Tenemos que detener la violencia = The violence of OTHER PEOPLE has to end
Here the verb dejar would be synonymous with abandonar:
Tenemos que dejar la violencia = OUR violence must end, we must abandon violence
The verb detener is also synonymous with arrestar (this last verb is rarely used):
La policía detuvo a tres criminales = La policía arrestó a tres criminales