It's important to remember that you cannot buy bus tickets on the buses in Bucharest. You must buy them beforehand from the grey ticket offices that are close to some stops. At these ticket offices, you can only pay in cash and you have to pay for the price of the card and the journeys that you wish to take.
Once you've got your ticket, you must validate it when you get on the bus by using the orange machines that are close to the bus doors.
The bus card costs 1.60 RON (US$ 0.40) and you can top it up by any amount you wish. A daily ticket costs 8 RON (US$ 1.80) on urban lines and 7 RON (US$ 1.60) per journey on the express lines that connect the airport to the city center.
The Bucharest bus system has many routes and stops which may be complicated for tourists, especially if you don't speak the local language. Having said that, it might be a useful option to get to the outskirts of the city or the airport.
Here is a map of the bus routes in Bucharest.