In the simplest form, you can think about social constructivism (Piaget, Zygotsky) as "learning by doing" and social cognitive theory (Bandura) as "learning by observing." Let's use a classroom setting to explain the differences. Classroom A has students that are being taught via social constructivist theory, and classroom B has students that are being taught via social cognitive theory. The teacher in classroom A will act as a mediator, encouraging the students to create their own hypotheses and find the answers, while the teacher in classroom B will instead try to explain everything to the students. Students in classroom A will be encouraged to think critically more than they memorize. Make sense?