Piaget was a Swiss psychologist who developed one of the most influential theories of cognitive development. He believed that children construct an understanding of the world through experiences interacting with objects and people. Piaget identified four main stages of development - sensorimotor, preoperational, concrete operational, and formal operational - defined by different cognitive abilities and ways of thinking at each stage. His theory emphasizes that learning happens through active construction and is influenced by biological maturation and interaction with the environment.