Constructivism is a theory of learning that suggests learners construct knowledge by interacting with their environment and experiences. It is based on the idea that learners build understanding based on what they already know. Key contributors to constructivism include Piaget, Bruner, Vygotsky, and Dewey. In a constructivist classroom, the teacher acts as a facilitator by seeking students' perspectives and allowing questions, while students work collaboratively and are actively engaged in learning through activities like projects.