Can someone give me a rundown of how to pay for public transportation options in Berlin? I will be doing most travel via foot, but from time to time will need to take a bus/tram or u-bahn. I will only need that for 2-3 (non-consecutive days). What is the best option on how to accomplish that? Are there travel cards or can you pay per trip? A basic overview would be very helpful.
Thank you.
PS -- are there buses, or only trams and underground?