Buenos Aires has an almost hopelessly vast, complex and extensive bus network. A few weeks of using it will probably give you a good idea of the basic framework but most tourists do not have that long, so they have to rely on outside help. Buses that run in the city are called collectivos and long-distance regional buses are called a micros. The Terminal de Omnibus de Retiro is the centre of the whole medley and you can find a bus that goes anywhere in Argentina from there. There are more than 75 platforms and multiple bus companies service each route/destination. There is general information in Spanish on the bus terminal website: http://www.tebasa.com.ar/paginas/buscador.html
Bus information line: 4310 0700
As long as you are travelling within the city limits there is a maximum ticket price of 1.25 pesos and tickets can be bought on a machine in the bus. Keep coins handy for exact change and learning a little directional Spanish will help.