A common noun is one which denotes a class of things, such as dog, pencil, boy, tree or book. It does not refer to a particular person or thing.
Solomon was a wise king.
Alice is a clever girl.
John is a boy.
In the above examples, Solomon, Alice and John refer to particular persons. These are called proper nouns. While king, boy and girl refer to the class or kind to which these proper nouns belong. These are called common nouns.
Common nouns include what are called collective nouns and abstract nouns.