Linguists define an appositive as a noun, or a phrase which includes a noun, which renames or clarifies a noun which appears beside it in the sentence. The appositive might appear as a short combination of words (even just an article and a single noun), or a more descriptive series of words (often including additional adjectives). The noun that the appositive describes generally appears either immediately before or immediately after the appositive.
Without an Appositive: The new graduate held his diploma.
With an Appositive: The new graduate, an owl wearing a purple mortarboard and academic gown, held his diploma.
Whenever you use an appositive in a sentence, whether the appositive appears at the very beginning of a sentence, in the middle of a sentence or at the very end of a sentence, you should separate the appositive from the remainder of the sentence using commas. If the appositive begins the sentence, put a comma after it; if the appositive appears in the middle of the sentence, usually just after the noun or subject, you should use a comma both before and after the appositive; if the appositive appears at the end of the sentence, ensure that you have added a comma before it. Commas may also appear within the appositive itself — for example, when two or more adjectives appear in the appositive.
Incorrect: A wide-eyed intelligent owl with a dream Gary had always wanted to graduate from school someday. He studied hard at Bird University a local college and finally realized his goals. Gary proudly put on his graduation attire, a violet mortarboard made especially for his feathered head.
Correct: A wide-eyed, intelligent owl with a dream, Gary had always dreamed of graduating from school someday. He studied hard at Bird University, a local college, and finally realized his goals. Gary proudly put on his graduation attire, a violet mortarboard made especially for his feathered head.