Preposition is one of the 8 parts of speech. They are the words that gives the positioning of noun, hence the root word postion(e.g.: in ,, around, at,about up ,down, over, under, above). Prepositional phrase is where a sentence starts out with the preposition and ends with a noun, pronoun, gerund (noun with -ing), or clauses (subecject+predicates). You can add a modifier (descriptive word) in betwee teh preposition and the ending noun, pronoun, gerund, or clause.