Realism, nominalism

For the last one hour I'm really stuck into these definitions. I couldn't decode their meanings. It would kind of you if you could throw some light on the issue.

nominalism
Philosophy The doctrine holding that abstract concepts, general terms, or universals have no independent existence but exist only as names.
[Am. Heritage Dic.]

nominalism
1 : a theory that there are no universal essences in reality and that the mind can frame no single concept or image corresponding to any universal or general term
2 : the theory that only individuals and no abstract entities (as essences, classes, or propositions) exist — compare ESSENTIALISM, REALISM
M-W's Col. Dic.]

realism
2 a : a doctrine that universals exist outside the mind; specifically : the conception that an abstract term names an independent and unitary reality b : a theory that objects of sense perception or cognition exist independently of the mind — compare NOMINALISM
3 : the theory or practice of fidelity in art and literature to nature or to real life and to accurate representation without idealization
[M-W's Col. Dic.]

realism
2. The representation in art or literature of objects, actions, or social conditions as they actually are, without idealization or presentation in abstract form.
3. Philosophy
a. The scholastic doctrine, opposed to nominalism, that universals exist independently of their being thought.
b. The modern philosophical doctrine, opposed to idealism, that physical objects exist independently of their being perceived.
[Am. Heritage Dic.]