Constructivism is a theory of learning that suggests students learn by actively constructing knowledge based on their experiences. There are two main types - cognitive constructivism, which focuses on individual understanding, and social constructivism, which emphasizes social interactions. Key theorists who influenced constructivism include Piaget, Dewey, and Vygotsky. In the classroom, constructivist teaching involves students exploring concepts through hands-on activities, asking their own questions, and collaborating with peers while teachers take on a facilitator role.