Indefinite pronouns are words that refer to non-specific people or things. Common examples include someone, anyone, everyone, something, and nothing. They are used when the exact identity of the noun is not important or is unknown. These pronouns can be singular or plural, affecting verb agreement. For instance, everyone is treated as singular, so you would say "Everyone is here." In contrast, several is plural, leading to "Several are missing." Understanding how to use indefinite pronouns helps in creating clear and concise sentences.