A preposition: is a part of speech (often called a function word) that is used to show the relationship between one noun or noun equivalent  (which is the object of the preposition)... to another noun in the sentence.

The prepositional phrase can behave like an adjective or an adverb.

Example 1:  The telephone is in the hall.   In this statement, the preposition in shows the relationship of its object hall to the subject (the telephone).  The prepositional phrase acts as an adjective in modifying telephone.

Example 2:  The telephone is ringing off the wall.  In this statement, the preposition off shows the relationship of its object wall to the subject (telephone) by modifying the verb. Here the  prepositional phrase acts as an adverb.

Prepositions can define spatial relationships as well as direction, location, time, and place.  They can also be used simply to introduce objects.  The resources at the bottom of the page provide essential details about these functions.   Although there are perhaps 150 prepositions, they represent less than one tenth of one percent of the English language.  Despite this fact, preposition are some of the most commonly used words in the English language.                 
                                        English Vocabulary: English Prepositions List:
                                     http://www.englishclub.com/grammar/prepositions-list.htm
The following are the most commonly used prepositions:   (This is not a complete list)