An appositive is a noun or noun phrase that renames another noun right beside it. The appositive can be a short or long combination of words. e.g.
The insect, a cockroach, is crawling across the kitchen table.
The insect, a large cockroach, is crawling across the kitchen table.
The insect, a large cockroach with hairy legs, is crawling across the kitchen table.
The insect, a large, hairy-legged cockroach that has spied my bowl of oatmeal, is crawling across the kitchen table.
Modifiers, on the other hand, are words, phrases, or clauses that provide description in sentences.
e.g.
Poor Stephen, who just wanted a quick meal to get through his three-hour biology lab, quickly dropped his fork on the cafeteria tray, gagging with disgust as a tarantula wiggled out of his cheese omelet, a sight requiring a year of therapy before Stephen could eat eggs again.
