A proper noun gives the actual, specific name of a person, place, thing or idea. While common nouns only refer to things in general, a proper noun is much more specific in what it refers to.
Proper nouns give the specific names of people, countries, companies, rivers and other landforms, streets, cities, etc. Proper nouns always begin with a capital letter. For example, "river" is a common noun, but "Mississippi River" is a proper noun, as it refers to an actual, specific river. As another example, "university" is a common noun, while "The University of Oklahoma" is a proper noun.