Remember that we ended up defining a presheaf as a functor. Given our topological space we set up the partial order category , flipped it around to so the arrows pointed the opposite way, and then said a presheaf of sets is a functor . So the natural home for them is the functor category , where the morphisms are natural transformations.
So what does this mean for our usual case where we consider presheaves of sets, or of sets equipped with some algebraic structure? Well, it means that we map from one presheaf to another one by picking a map for each and every open set: . But these maps must be compatible with the restrictions: if then we must have . That is, given an element in , we can either first restrict it to and then map it by to , or we can first map it by to and then restrict the result to . In either case, we should get the same answer.