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

A proper noun is a name of a person, place, thing, or idea. A common noun is a name of a person, place, thing, or idea that is not specific.

For example, "John Smith" is a proper noun, while "the library" is a common noun.

Proper nouns are always capitalized, while common nouns are always lowercase.

Proper nouns are often used in formal writing, while common nouns are often used in informal writing.