The Mayans are known for the only known fully developed writing system of the indigenous Americas, as well as for its art, architecture, and mathematical and astronomical systems.
The Aztecs were famous for their agriculture, mountain terraces, introducing irrigation, and creating artificial islands (chinampas) in the lakes. They also created a form of hieroglyphic writing, a calendar system, and constructed famous pyramids and temples.
The Incas were a small tribe in the Andes Mountains of Peru. In a very short amount of time, they built a very large empire with advanced engineered roads, walls, and irrigation projects, which are still used today.