A noun is generally known as a person, place, thing, or idea.  Examples are words like boss, train, car, freedom, dog, etc.

A noun phrase, however, refers to a group of words, or a phrase, that modifies that noun.  These modifiers can include words like determiners (a, the, some, etc.), adjectives (tall, yellow, smart, etc.), and, of course, the noun.  Other parts of speech can be part of the noun phrase also, such as adverbs and prepositional phrases.

Some examples are "The yellow car...", "Three blind mice...", and "Many of my best friends from college...".

A noun phrase and a verb phrase make a complete sentence.  A noun phrase on its own, obviously, is not a complete sentence because it does not complete an idea.