For some people, there is confusion over constructivism. For starters, constructivism is not considered a theory by many educators. Rather, constructivism is a philosophy that addresses the nature of knowledge and learning.
Constructivist see knowledge as always changing and being developed by the learner and is built upon the work of Piaget, Vygotsky, and even Dewey. In this philosophy, the learner develops knowledge by building upon what they already know. The learner is actively involved in their learning as they interact with their environment and with other people. In behaviorism, it is an external force that acts upon the learner but in constructivism it is the learner who is acting upon the external environment. The student transforms the knowledge as the internalize it.
A curriculum that is heavily influenced by the philosophy of constructivism has students who are actively engaged in learning in a social environment. This includes such strategies as project based learning, cooperative learning, and opportunities for problem solving. For many, including opportunities for reflecting on learning experiences helps students to build knowledge is another aspect of constructivism.