Prepositions are words that connect nouns, pronouns, or phrases to other words in a sentence. There are two types: simple prepositions, which are single words like "in" and "on", and compound prepositions which are multiple words like "instead of" and "by the side of". Prepositions also indicate place, time, or direction and are used with verbs, adjectives, and other parts of speech. Common examples of different types of prepositions and their uses are provided.