Short answers, pulled from the story.
Vygotsky is best known for his work on psychological development in children and for creating cultural-historical activity theory. He introduced the zone of proximal development, the concept of scaffolding in learning, and the argument that higher mental functions emerge from social and cultural interaction rather than from biology alone.
The zone of proximal development is the gap between a child's current independent level of ability and the higher level they can reach with guidance from a more knowledgeable other. Vygotsky argued this zone provides a prospective view of cognitive development, showing what a learner is capable of becoming, not just what they can do alone.
In 1936, the Communist Party condemned pedology (developmental psychology), and Vygotsky's theory was caught in the resulting crackdown. His work remained banned until after Joseph Stalin's death in 1953, with a first major collection of his texts published in 1956.
Piaget believed egocentric speech preceded inner speech and dissolved away as children matured. Vygotsky showed the opposite: egocentric speech becomes inner speech, which is then what we call thought. The two never met, though Vygotsky had read a number of Piaget's works.
In one of his last private notebook entries, Vygotsky wrote: "This is the final thing I have done in psychology - and I will like Moses, die at the summit, having glimpsed the promised land but without setting foot on it. Farewell, dear creations. The rest is silence." He died on the 11th of June 1934, at the age of 37.
Vygotsky viewed play as the leading activity of preschool-age children and as the primary source of their psychological development. He saw play as the best arena for children to practice mediation, since it requires them to follow social rules voluntarily, which builds the self-regulation underlying higher cognitive functions.