Preposition means a word that is in a fore position. Prepositions, therefore, generally precede those words that they modify.
The most familiar prepositions describe spatial relationships between two or more things; these include under, on, atop, behind, before and alongside. Other prepositions include with, without and except.
English has prepositions, such as with, against and after, that consist of only one word. Some prepositions, like "with regard to," "on top of" and "in front of," are themselves phrases.
A preposition and its complement, a noun or noun phrase, combine to form a prepositional phrase. This, according to Heather MacFadyen of the University of Ottawa, creates an adverb describing where something happened.
Many consider it to be grammatically incorrect to end a sentence with a preposition in English because prepositions much logically come in front of something. However, it is common English usage to place prepositions at the end of a sentence, which may be an example of postpositions, words that have the same purpose but follow the words they modify.