A possessive is a grammatical form that shows ownership or relationship between nouns. It often involves adding an apostrophe and the letter "s" to a noun, such as in the phrase John's book, indicating that the book belongs to John. For plural nouns that already end in "s," only an apostrophe is added, as in the dogs' owner. Possessives can also be used with pronouns, such as his, her, or their, which indicate ownership without needing an apostrophe. For example, in the sentence "This is her car," the word her shows that the car belongs to her.