A preposition relates or 'positions' a noun or pronoun to another element of the sentence. Common prepositions are words such as 'at', 'by', 'in', 'to', 'from', 'under', and 'with':
We all went to the cinema straight from the restaurant.
Prepositions can have a direct relationship with the physical world, or relate abstract concepts to each other:
The Conservatives have slid to the left of the Liberals.
The following list of common prepositions appears in the Webster's Encyclopedic Dictionary of the English Language:
above, about, across, after, against, among, around, at, before, behind, below, beneath, between, but, by, down, during, except, for, in, inside, into, like, near, of, off, since, to, toward, through, under, until, up, with, and within.
Usage can be more of an art than a science. Often it comes down to what sounds right rather than the application of a rule.