The document explains Vygotsky's social interactionist theory, emphasizing that cognitive development arises through social interaction and is supported by instructional scaffolding. Vygotsky proposes that learning precedes development, contrasting with Piaget's view that development must occur first. Key concepts include the 'more knowledgeable other' who aids learning, the 'zone of proximal development', and the classification of linguistic development into social, private, and silent speech forms.