What is the difference between a common noun and a proper noun?

A common noun is the general name given to a person, place, thing, or idea. A proper noun is the specific name of a particular person, place, thing, or idea.

Common Nouns		Proper Nouns 
doctor			Dr. William Sharp

avenue			Third Avenue

volcano			Mount St. Helens

a theory		Pythagorean Theorem

Common nouns are not capitalized, while proper nouns are.