The main point of the nominalist position pertains to the relationship between ideas (universals to be more precise) and reality these ideas represent. It maintains that ideas are a product of human consciousness and do not exist in reality. It follows that they cannot cause anything in the realm of reality. In other words, altering depictions of reality (ideas) do not produce any changes in the reality itself. The realist position, by contrast, maintains that ideas have real existence - they are more real than empirical experiences of reality. http://en.wikipedia.org/wiki/Problem_of_universals
---- I always thought this was a misleading distinction. For one thing, one can't actually speak of ideas; one can only speak of language. The nominalists plumped for the aritrariness of signs; the realists for a hard connection between signs and referents.
But the deeper connections language makes is not between signs and referenents, but between speakers. So the reality of language has a lot more to do with its power to shape consciousness and motivate action than it does with its actual connection to referents.