Everyone knows about it. Everyone has heard about it. Everyone has seen pictures of it. The once-hidden core of the Incan empire is now a hub of commerce and activity, prompting the government to take an active role in regulating the number of visitors to it and the Inca Trail leading to this historic site.
Macchu Picchu is the most popular tourist destination in South America for a reason: the views are spectacular, it's a manageable size, and there are plenty of services for tourists in the nearby staging town, Aguas Calientes.
Cheap lodging, good restaurants, Internet connections, and shops cram the small hamlet perched on the edge of the Urubamba River. When departing the train from Cusco, the traveler is bombarded by locals from the town selling film, panchos, T-shirts... just about everything the typical gringo needs.
Macchu Picchu itself is a 20-minute bus ride from Aguas Calientes, up a narrow and winding road. From there, it's a $20 entrance fee, and you're set to wander in the footsteps of the Incans. Those who have avoided the effects of the altitude can take the 45-minute climb up Huayna Picchu, the giant mountain that looms over the city of Macchu Picchu and graces countless pictures of the site.
The site itself is completely "doable" in one full day without feeling rushed, although if Huayna Picchu is in your plans, perhaps a day and a half is a better plan, because when you return from the mountain, you probably won't feel like doing much else. If you're energetic, not interested in soaking in every last detail of Macchu Picchu, and an early riser, though, one day should suffice for everything.