Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They are typically placed before the noun or pronoun they modify. Examples of prepositions include "on," "in," "at," "under," and "beside." Postpositions, on the other hand, are words that show the relationship between a noun or pronoun and other words in a sentence, but they come after the noun or pronoun they modify. Postpositions are used in some languages, but they are not commonly used in English. An example of a postposition is "ago" in the phrase "two hours ago."