Machu Picchu is impossible to describe. No pictures or words can really describe it. Here's the basics:

The citadel of Machu Picchu, meaning “old peak” in the native Quechua language, is named for the mountain on which it sits. It looms 2,000 feet above the serpentine Urubamba River in the tropical mountain forest of the upper Amazon basin. The city’s finely hewn granite blocks comprise dwellings, agricultural terraces and storehouses, plazas and temples. It is recognized as a feat of architectural planning, engineering and stonemasonry and was built without the use of iron tools or draught animals.
 
Machu Picchu was constructed around 1450, at the height of the Inca empire, and was abandoned less than 100 years later, as the empire collapsed under Spanish conquest. Although the citadel is located only about 50 miles from Cusco, the Inca capital, it was never found and destroyed by the Spanish, as were many other Inca sites. Over the centuries, the surrounding jungle grew to enshroud the site, and few knew of its existence. It wasn’t until 1911 that Yale historian and explorer Hiram Bingham brought the “lost” city to the world’s attention. Bingham and others hypothesized that the citadel was the traditional birthplace of the Inca people or the spiritual center of the “virgins of the sun,” while curators of a recent exhibit have speculated that Machu Picchu was a royal retreat. Regardless, the presence of numerous temples and ritual structures proves that Machu Picchu held spiritual significance for the Inca.