Colectivo
Colectivo is basically the bus in Buenos Aires. 2 pictures: One on the street and one at a bus stop. You can get everywhere in the city with the colectivo. The subway is a good means of transport but if you have to go to a place where no subway station is near the colectivo is a good alternative
You pay in the bus on a machine which only accepts coins. No coins, no ride. They don't change. So what everyone does is save their coins for the bus. The normal amount is 0,80 $ or 80 Centavos. For this you can travel from one stop to the end of the line. There is also a reduced ticket which costs 0,45 $. Children when going to school pay 0,05 $. For that they have to wear their school uniform or pay the regular 0,80 $.
Be prepared to stand. Often there are no free seats available. this depends on where get in the bus and also on the time. Obviously rush hour is most certainly standing. And they pretty much fill up the bus too. As long as the door closes! That is not really fun a ride like that.
I prefer the subway if i can. 1. it's cheaper, 2. You don't need to have coins, 3. Is often quicker, above ground traffic can delay.
If you want to read more check Wikipedia.