Machu Picchu is the most well-known Inca ruins around and the most visited tourist attraction in Peru. It's about 50 miles northwest of Cusco and is on a mountaintop at 2,400 meters. Machu Picchu is actually the name of the mountain on which the ruins sit (picchu mean peak in Quechua).
If you want to visit this site, you'll start at Cusco. There were more tourists in Cusco by far than I had seen anywhere else in my 9 months' of travels. Still, it is a nice city with the usual plazas and Spanish-built churches. It also has some amazing walls built of really big rocks that have edges that fit together perfectly. How the Incas machined these rocks to achieve this kind of fit is astounding.
If you want to hike the Inca trail—a four-day affair—you have to schedule it about three months in advance. There are also other options for hiking to the ruins. If you're into hiking, check it out way ahead of time.
The more common alternative is to take the train or a bus to Aguas Calientes, a small town within a stone's throw of the ruins. There is a train that goes all the way to and from Aguas Calientes from Cusco, but it's slow and kind of expensive. A quicker and cheaper alternative is to take the bus, or better yet, a cab, part way, to Ollantaytambo, then the train the rest of the way. Go to the bus station and talk to a cabbie on the street. Within a few minutes, he'll have rounded up four people and you can go straight to Ollantaytambo (the bus will go to Urubamba, then you'll have to catch another one to Ollantaytambo) for 10 Solis. Book the train from Ollantaytambo to Aguas Calientes and back a couple days ahead of time at the train station—or online.
From Aguas Calientes, you'll want to rise early. You can either hike up a thousand stairs to the site, or you can take the bus. The day before you go up, you should buy the entrance ticket and a round-trip bus ticket to the ruins in Aguas Calientes. The first 400 people in the ruins have the option of climbing Huayna Picchu, the big mountain behind the ruins. Make sure to take the path to the right after you enter the ruins. The one to the left simply goes up some grueling switchbacks and will take you much longer to get to the entrance to Huayna Picchu—there's a hut at which you sign in and get let into the trail leading up the mountain.
The view from the top is fantastic. There are some ruins on top of this hill, too. How these people did this is baffling. The terrain is steep and they worked with big rocks. Makes me wonder how many of them died while building these sites. There are lots of opportunities for one to fall and die while doing this hike. If you're afraid of heights, you probably won't make it to the top. If you're not, you have to do it. Once on top, the view is worth the exertion—if it's not clouded in and raining.
Something funny? When we were on our way down—I climbed Huayna Picchu with Rich from Seattle and Julie from Canada—we saw one guy going up wearing a heavy parka and another guy wearing flip-flops. Psycho, both!
After you enter the site, Huayna Picchu is on the far side. On the near side—near the top of the switchbacks by the entrance—is a trail leading to the Sun Gate. The Sun Gate is where those hiking the Inca Trail will first glimpse the site. If it's clear, you can see the Sun Gate from the site. You can hike there for a great view in about an hour.
If you've got time, I'd recommend spending a couple nights in Aguas Calientes, and taking the first day to walk around the ruins. A private tour guide will give you a better understanding of the ruins if you've got a couple extra bucks to spend. Take the second day—the morning is really all you need—and climb Huayna Picchu. For me at least, after climbing Huayna Picchu, I was pretty wiped out and would've enjoyed the ruins more if I'd had more energy. If you're pressed for time, however, like me, the whole ball o' wax is doable in one day.
Note: I stayed at a hotel that is cheap and clean. The owner was also really nice. It's called Hospedaje California. When I arrived in town, the train pulled up on the lower track (below the main track and train station), next to a row of stores and hotels, and near a couple banks, a stone's throw from the bridge. This is where it is located. I recommend it.