We were used to pay tickets with coins because the ticket used to cost less than $1 (argentine peso), but nowadays coins are useless because of the devaluation, the government decided to install a new system to pay transportation in the city. This is when the SUBE card was invented, a card to pay buses, subways, trains and even highway tolls.
For tourists, this system was not very comfortable because at first you could not get the card in many places, but now you can buy it in every district of the city. Here you will find a map where you can get a SUBE card.
The cost of the card is AR$25 and there is no cost to recharge it.
The transportation system is being expanded and the idea is to use SUBE in other cities of Argentina. This is the case of Bariloche, where you can travel by bus with the same card that you get in Buenos Aires.
Check this post about how to use public transport in Buenos Aires!