A preposition is a word which precedes a noun (or a pronoun) to show the noun’s (or the pronoun’s) relationship to another word in the sentence. (The word preposition comes from the idea of being positioned before. It is not true to say that a preposition always precedes a noun or a pronoun, but it does most of the time.)
The following are all prepositions:
above, about, across, against, along, among, around, at, before, behind, below, beneath, beside, between, beyond, by, down, during, except, for, from, in, inside, into, like, near, of, off, on, since, to, toward, through, under, until, up, upon, with and within.