A noun is a person, place or thing. The thing does not have to be a physical object. A noun can be an abstraction such as an idea or a belief, or it can be a gerund (A participle used as a thing). An example of a gerund as a noun  would be:

Learning is worth the effort. 
What is the subject of the above sentence? Learning. That is a gerund. It is a special type of noun. 

Nouns as Phrases

A noun phrase is defined as a group of words functioning as a noun that DO NOT contain a subject/predicate pair. For example:

My nieghbor's dog barks too much.

Beautiful blue skies and the  bright orange sunsets prove that nature is a work of art.

The green text all contain nouns along with other words that modify those nouns. The group of works, however, do not contain a subject/predicate pair.

If a noun group of words contains a subject predicate pair, then it is a noun clause. 

Nouns as Clauses

A noun clause is a clause with a complete subject/predicate pair that can act as the subject or object of the verb. The above sentence contains a noun clause.

Beautiful blue skies and the  bright orange sunsets prove that nature is a work of art.

The green text is the object of the verb - proves. It is a noun clause because it acts as a noun and has a subject/predicate pair.

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