A noun is a proper noun if it refers to a specific person, place, thing or idea and requires capitalization. On the other hand, if a noun does not require capitalization and refers to a generalized, non-specific person, place, thing or idea, it is a common noun.
Proper nouns always require capitalization of the first letter. Days of the week, such as Monday, are proper nouns, for example. Proper nouns also describe months, such as October; people's names, such as Mary; and city names, such as San Francisco. Proper nouns are also necessary to describe street names, countries and brands.