La Catedral is actually three churches in one, each of which are exquisite. It is decorated in the Baroque style, with more gilt and gold trimming than you can imagine. In order to convert the Incan people to Catholicism, the Spanish used symbols from Incan religion throughout the church. Look for the hundreds of mirrors, as well as the triangle shape associated with the Mountain God.
La Catedral hosts a couple of particularly unique pieces, including a Peruvian rendition of the Last Supper featuring a guinea pig as the main dish. El Señor de Los Temblores is a crucifix that stopped the Cuzco earthquake of 1650, and is featured prominently in the cathedral.
Pictures are not allowed inside, but the exterior of La Catedral is picturesque both during the day and at night when it's lit up with the rest of La Plaza de Armas.