Both R20 and PAR20 bulbs have a high light intensity compared to traditional halogen lamps. Because they are the same size in diameter, they can easily be swapped out for one another. R bulbs, which stands for reflector light bulbs, have a wide flood beam angle that make them great for illuminating large areas.
PAR bulbs, on the other hand, stand for Parabolic Aluminzed Reflector light bulbs and offer a spot light. PAR bulbs are ideal for outdoor security lights and indoor track lighting. A good rule of thumb is that PAR bulbs provide more focused light, and R bulbs provide a more diffused light.