The Ancestral Pueblo, popularly referred to as the Anasazi and sometimes as the Hisatsinom, were a Native American people who lived in advanced structures near the "Four Corners" region where Arizona, Utah, Colorado, and New Mexico intersect from about the time of Christ's birth to 1300 AD, when they began to move away for reasons that are still not clear today.  (Anasazi is a Navajo term meaning "the ancient ones" or "ancient enemies", so the term "Ancestral Pueblo" is preferred by their direct descendants, the Pueblo people. After abandoning their advanced structures, they lived as hunter-gatherers for awhile, but again began to build advanced villages along the Rio Grande in New Mexico, around 490 AD.
The Anasazi were highly influential to Native American and New Mexican architecture. Many lived in circle-shaped pitlike structures with a sipapu - a hole in the ground. Others lived in carved out buildings inside of caves. The former went on to influence the structure of the Pueblo ceremonial room known as the kiva. A kiva always contains a sipapu, and the Pueblo believe is that this is where their ancestors entered the world.  The Anasazi were also known for their horticulture - which was not easy for them, given the arid climate in which they lived - and their pottery. 
A common misconception is that the Anasazi completely disappeared, and this is not quite accurate, as their descendants survive to this day. However, liberal scientists believe their status as a "continuing culture" vanished. Though most real-world sources agree that several structures, like Sky City of the Acoma Pueblo and the apartment complexes of Taos Pueblo, to be much older and continuously inhabited than currently dated.
- Appiah & Gates, The Dictionary of Global Culture, Alfred A. Knopf, New York 1997