Msgr. Charles Pope gives the Vile Error of Nominalism a relatively simple exposition:
How have we gotten into this mess wherein we have set aside reality in favor of what we think reality is? No longer do we go out to meet reality and accept the obligation of conforming to reality; now we sit back and claim the right to posit our own reality, to project reality and define it on our own terms. How did we get here?
Look to the nominalists, my friends.
A rather informative, though challenging, book on this matter is Journey to Modernity by Louis Dupré. In it he traces the medieval synthesis and rise of nominalism in the late 15th century, which in turn gave way to the Cartesian Revolution in the 17th century [and laid the groundwork for the Protestant schism].
I preached against Nominalism in my homiletics class this morning! In essence, nominalism is the de-sacralization of western language, the stripping from our most cherished concepts of any transcendental referent.
It's Evil. And it's the best reason we have for our future priests to learn Thomism.