A determiner is a word that introduces a noun and specifies its reference in context. Common types of determiners include articles (like "a," "an," and "the"), demonstratives (such as "this," "that," "these," and "those"), possessives (like "my," "your," and "his"), and quantifiers (such as "some," "many," and "few"). Determiners help clarify whether a noun is definite or indefinite, singular or plural, and they play a crucial role in sentence structure.