Realism is a theory that universals (properties such as ‘redness’) have an existence independent of the human mind. Realists hold that the essence of things is objectively given in nature, and that our classifications are not arbitrary. As such, realism is contrasted with nominalism, the theory that universals are merely names or general terms.
More generally, realism is any philosophical theory that emphasizes the existence of some kind of things or objects, in contrast to theories that dispense with the things in question in favour of words, ideas, or logical constructions. In particular, the term stands for the theory that there is a reality quite independent of the mind. In this sense, realism is opposed to idealism, the theory that only minds and their contents exist.
Realists believe that theories are not just instrumentally useful, but also tell us something about the nature of reality.
Anti-realists believe that theoretical entities (such as genes, atoms, or the factors derived from factor analysis) are "splendid fictions." Theories are useful instruments for prediction, but they do not tell us anything about the underlying reality of nature.

Read more from: http://plato.stanford.edu/entries/realism/