Prepositions are words used before nouns or pronouns to show their relationship to other words. There are three main types of prepositions: location, direction, and time. Prepositions of location indicate where something is, like "on" or "in". Prepositions of direction show movement, such as "to", "from", or "over". Prepositions of time specify when something occurs, like "by", "for", or "after".