Both Essentialists and Relativists function under the greater theology of philosophy.
Essentialists tended to believe that for any object (or idea) the object must possess specific characteristics which defined the idea or object as being a part of something, or definable, in a very specific way. Therefore, these objects can be defined by very universally understood and accepted characteristics.
Noted Essentialists were/are Plato, Aristotle, Diana Fuss, and Susan Gelman.
Relativists tended to believe that all things are relative based upon ones own personal morals, thoughts, and perceptions. Basically, relativists believed that no absolutes exist and ideologies are created through personal interpretations and justifications.
Noted Relativists were/are Mahavira, Socrates, Bernard Crick, Paul Feyerabend, Joseph Margolis, and Friedrich Nietzsche.