A Common Noun names any regular, ordinary person, animal, place, thing, or idea. Nothing specific.

superhero, river, holiday, religion, month, day, city, composer, boy, car, language

A Proper Noun names a very specific, very particular person, animal, place, thing, or idea. A Proper Noun always begins with capital letter (unless it is a brand name like eBay or iPad).

Batman, Mississippi River, Fourth of July, Buddhism, December, Monday, Ludwig van Beethoven, Peter, Volvo, London