The Sambodromo is a purpose-built venue designed by famed Brazilian architect Oscar Niemeyer for the sole purpose of the competition of various samba schools. Each samba school, simply an organization which puts together a group of floats, dancers, and performers, comes from a favela, or slum. They spend most of the year constructing floats and preparing costumes—they also spend millions of dollars. The competition takes place over the official four days of Carnaval, with the final competition on the following Saturday.
The Sambodromo is in a surprisingly crummy and filthy neighborhood. Moreover, it is not what one would have expected from its architect, considering some of his other amazing work. It's basically a big, ugly block of concrete.
Regardless, an evening in the Sambodromo is something you won't soon forget. The floats, costumes, and performances are incredible—nay, astounding. You'll enjoy it from 9PM to about 5:30AM. Each of six schools performs for eighty minutes. Buy a ticket early in the year and get a good seat. It's worth the money. The price of the tickets goes up as Carnaval approaches.