Not much is known about the people that built the city of Teotihuacán. During it's height it was one of the most important cities in MesoAmerica, holding some 200,000 people and forming the largest and most sophisticated city in the Western Hemisphere. However, around the 7th Century, the city was abandoned. The pyramids, citadels, temples and palaces remained unseen until the Aztecs arrived around 1200 AD.
The Aztecs named this ancient city 'Teotihuacán' which in Nahuatl means "place of those who have the road of the gods". and believed that the earth, sun, moon and universe were created here.
Today, Teotihuacán (pronounced teotiwa'kan) still has many of the structures that were standing when the Aztecs arrived and is a great destination being only a 40 minute bus trip from Mexico City.
The Avenue of the Dead. Receiving it's name from the Aztecs who believed that the buildings that line the avenue housed the graves of giants who became gods. These houses are now believed to be residential complexes.
Pyramid of the Sun - One of the largest pyramids in the world.
Pyramid of the Moon - One of the principal ritual areas of the ancient city.
For a great insight into the city check out the Wikipedia Website:

It was a fantastic experience. We arrived very early before the tour buses arrived and literally had the whole place to ourselves. The steps up the Pyramids are extremely steep and require that you hold onto the railing so that you don't fall. We had a great time here. A lot of the stonework was intricate with small black or colored stones in the cement binding the stones together. Also shown is the mural of the Puma with the color still showing found by archaelogists. Eagles, falcons and cats seemed to be very important to the culture that built this city.