Pronouns are words that replace nouns in a sentence, helping to avoid repetition and make sentences clearer. Common pronouns include he, she, it, they, and we. For example, instead of saying "Maria likes Maria's book," you can say "Maria likes her book," using the pronoun her to refer to Maria. Pronouns can be categorized into different types, such as personal, possessive, reflexive, and demonstrative. Personal pronouns refer to specific people or things, while possessive pronouns indicate ownership, like his or their. Understanding pronoun usage is essential for effective communication and writing.