Devices that obey Ohm's Law are linear devices. Diodes are non-linear. A perfect diode actually funcations like a switch - turning on and off. The switching action of diodes if of fundamental importance in electronics. This sensitivity to the polarity of a signal finds a multitiude of uses.
Diodes allow a current to flow one way, preventing current from returning back.
However, if enough electrons builds up, a avalaunch voltage may occur - when the electrons just breakthrough the dam holding all the water and flood backwards. Capacitors are like batteries. Electrons will build up at one end of the plate until PE is so great electrons start jumping. DIodes simply allow currrently to flow only in one direction, unless so much energy is built up on the other end that a avalanch voltage sistuation occurs. Theoretically a diode will not let a single amount of current the other way, while resistence is none while there are some in reality.