The main problem that makes the difference between nominalism and realism is the question of the existence of properties beyond objects. Realism in its radical form, or Plato’s realism, refers to the universals as independent entities that exist regardless of the objects in an ideal world. Moderate realism, which is also called Aristotelian or strong realism, rejects Platonic realism. According to this point of view, universals are real but exist only when an object having certain properties exist. Nominalism, in its turn, denies the existence of universals. Thus, nominalism and realism are two opposite views on the nature of universals.
To my mind, Aristotle’s realism is one of the best concepts for the description of our reality. On the one hand, we live in a world that consists of material objects. On the other hand, everyone is able to discern certain properties of these objects. Every individual knows what length, shape, or temperature is. Besides, the existence of numbers and mathematical principles allows us to suppose that there is some realm existing between material and ideal worlds. On the one hand, numbers do not exist, but on the other hand, mathematics successfully describes real physical objects.
Being a Christian, I consider that universal ideas and properties were created by God, but they can exist only in the objects of the material world. It could be argued that there are objects with the same properties which do not have anything in common. For example, a beautiful girl does not have anything in common with a beautiful sunset. However, not all the members of a certain category should necessarily have common traits, and this argument does not prove that there are no universals at all.
I see the point in bundle theory developed by Mill and advanced by Hume. According to this theory, there is nothing but a bundle of perceptions of objects related only by causation and resemblance. However, “resemblance” is a universal idea itself, as could be seen from the fact that people can see common attributes in things that actually do not have anything in common. Thus, when nominalists take to attempt to prove their point of view, they use universals. It allows suggesting that realism can successfully describe our world.