Pronouns are words used to replace nouns in a sentence, making communication clearer and less repetitive. They include personal pronouns (I, you, he, she, it, we, they), possessive pronouns (mine, yours, his, hers, ours, theirs), reflexive pronouns (myself, yourself, himself, herself, itself, ourselves, themselves), and relative pronouns (who, whom, whose, which, that). Pronouns help avoid redundancy and are essential for effective writing and speaking. Understanding their proper use is crucial for mastering English grammar.