Auburn University
Auburn University: A Glossary of Political Economy Terms: Complementary Goods
This site gives a good definition of "complementary goods" and provides examples. It also explains how when the price of Good A changes, the demand curve for Good B will shift.
Auburn University
Auburn University: A Glossary of Political Economy Terms: Substitute Goods
Dr. Johnson thoroughly explains the concept of "substitute goods," and how a price increase of one good will shift the demand curve for a substitute good.