Excuse the schoolboy Latin, but Peru, like Gaul, is divided into three parts: the coast, the mountains, and the jungle. In Spanish, that comes out as the Peruvian schoolkid mantra costa, sierra y selva. As you can see from the above map, the narrow coastal strip is the smallest of the three—and by far the most populated. It contains the largest cities, including the capital Lima. It is also the driest, being a northern extension of the Atacama Desert, where rainfall does not, for all practical purposes, ever occur.
When we think of Peru, we generally think of the Andes, which takes up the second largest chunk of Peruvian territory. Here are the tourist meccas of Cusco, Machu Picchu, and Lake Titicaca, as well as several isolated mountain metropolises like Arequipa, Huancayo, and Ayacucho. The locals here speak mostly Quechua and Aymara. This is the second most populated region.
Finally, there is the jungle. The mighty Amazon has its source in rivers flowing into the Marañon and Ucayali River systems from various parts of Peru, Ecuador, and Colombia. Here is two-thirds of the total land area of Peru, but only 11% of its population. Culturally, it is one of the most interesting parts of the country, but many (including myself) are deterred by Yellow Fever, Malaria, Dengue, and a whole host of tropical diseases.
If I went to Peru, I would concentrate on Lima and the high country between Lake Titicaca and Machu Picchu. (That is, if I don’t develop severe soroche. If I do, I might take a side trip to Northern Chile via Tacna and Arica.)