Determiners are words used with nouns to provide information about quantity, reference, or attributes. They clarify nouns by defining, identifying amounts, or stating possession. The main types of determiners are articles, demonstratives, possessives, and quantifiers. Articles like "a" and "the" and demonstratives like "this" and "that" are used with both singular and plural nouns, while possessives and quantifiers specify ownership or amounts.