Determiners are words that come before a noun to give more information about the noun. They include articles (a, an, the), demonstratives (this, that, these, those), possessives (my, your, his, her, its, our, their), quantifiers (some, any, many, few), and numbers (one, two, three). They help to specify or quantify the noun. For example, in the sentence "I have three books," the word "three" is a determiner that specifies the quantity of the noun "books."