A Prepositional Phrase is a modifying phrase consisting of a preposition and its object. Some examples of Prepositional Phrases are:
To, Up, Down, In, Out, Around, By, Besides, With, Without, Through, Past, Over, Under, Underneath, Until.



Prepositional Phrases are either Adjectives or Adverbs.
When they are Adjectives, they modify nouns and pronouns, they answer these questions:
Whose?, Which one?, Number?, and What Kind?.
When they are Adverbs, they modify verbs, adverbs, and Adjectives, they answer these questions: How?, When?, Where?, and Why?


Links:
1. http://www.oestarapublishing.com/grammar/prep.html
2. http://www.yourdictionary.com/grammar-rules/Prepositional-Phrases.html
3. http://www.writingcentre.uottawa.ca/hypergrammar/preposit.html
4.
http://www.leasttern.com/Grammar/PrepostionalPhrases1c.htm
5.
http://esl.about.com/library/quiz/bl_prepphrase1.htm