Constructivism is a learning theory that suggests learners actively construct knowledge from their experiences through sensory input. There are two main types: cognitive constructivism focuses on individual internal construction of knowledge based on Piaget's theory, while social constructivism sees knowledge as initially shared and based on Vygotsky's view that learning is social. Key principles of constructivism are that learners construct their own meanings, new learning builds on prior knowledge, and learning is enhanced through social interaction and authentic tasks.