Carnaval in Salvador basically has two parts: the parade of trio elétrico and the barracas. A trio elétrico is a done-up semitrailer, loaded with sound equipment and with a band playing on top.
They parade very slowly along one of two circuits; one closer to the city centre, running from Campo Grande (literally "Big Field", Salvador's central park) to Praça Castro Alves (named for Antônio Frederico de Castro Alves, the Bahian poet who, among other things, wielded his mighty pen against the injustices of slavery and political oppression) and the other running from Barra to Ondina, along the Atlantic Ocean. They are called "trios" because the first one was an old car ('29 Ford) with a driver and two musicians (Dodô and Osmar) in the back (the car can be seen in the museum at the Lagoa da Abaeté in Itapoan; it debuted in 1950).
The trios form the nucleus of the blocos. One pays to join a bloco and is given an abadá (a getup consisting of a t-shirt and shorts, usually), which allows one to parade with the bloco inside the cordão (rope carried by security personnel). The people who aren't in blocos, and who are hence outside of the roped-off areas around the trios, are called pipoca (or popcorn).The other part of Carnaval is the barracas. They are everywhere, turning Salvador into a city of ten thousand parties. A lot of them have their own sound systems. And where there isn't a barraca, there'll be somebody with an isopor (styrofoam cooler) selling beer or batidas (cachaça/fruit mixtures; killer strength).
On the Thursday evening which is the beginning of Carnaval, the city's mayor turns the key to the city over to Rei Momo at Campo Grande (rei is "king", and although " Rei Momo" is a different person every year "he" always looks like an overweight Nero). Thursday is generally kind of a slow Carnaval night ("slow" is a very relative term here), a lot of people still have to get up and go to work on Friday. Friday night picks up, and then on Saturday (Sábado do Carnaval) all hell breaks loose.