Realitivity is Einsteins improvement on Newtonian mechanics. Newtonion mechanics does not deal with the effects on an object at the speed of light where as Realitivity does. Such features of the Theory of realitivity are the eqvialence principal i.e. acceleration is the same as gravitation and then there are Lorenzt transformations. Qantum mechanics is like infinitesimal mechanics, it deals with the mechanics of the atom, the electron(an example of a principal from QM is Heisenbergs uncertainty principal) but however unlike real life these cannot be observed and so thats why probability plays a huge part and thats why Einstein himself tried to produce a unified field theory to extinguish the uncertaintys that existed in Quantum mechanics. However he failed and the heir so to speak to realitivity is string theory.