A noun is a person, place, thing or idea. Nouns come in many forms -- singular, plural, common and proper. An appositive is another form of a noun and is a word or phrase that renames another noun in a sentence. Using appositives is one way to add details to your writing. There are two different types of appositives -- non-restrictive and restrictive.
A non-restrictive appositive is one that provides helpful information, but is not necessarily required for understanding. For example, in the sentence, "My husband, an accountant, just got a new job," knowing that my husband is an accountant gives details to what type of job he got, but it could be removed without changing the meaning of the sentence. Non-restrictive appositives need to be surrounded by commas.
A restrictive appositive provides crucial information and should not be set off by a pair of commas. In the sentence, "The worker who stole money from the cash register was fired," the phrase "who stole money from the cash register" is the appositive phrase. It is necessary information to explain why the worker was fired.