Nominalist- rejects the idea of universals (individual cases and particular exhibitionism of some trait may exist, but the trait cannot be tangibly defined by a universal definition; partially because universals are often relative (hot vs. cold, for example) and are defined upon a spectrum)
Realist- accepts the idea of universals (universals are real and are the entities which one describes (eg. classification of objects))
Sorry if my explanation is rambly. Wiki probably explains it better.