In my opinion, besides getting you to your destination, the train rides themselves can be very enjoyable and beautiful. Busses are very useful, and I rode many. They are convenient to get around cities. Usually if you are buying the bus ticket from the driver you can get change, and the driver will let you know how much to pay, since the cost will depend on the destination. Some busses use the honor system, where you are supposed to buy a ticket, then validate yourself onboard, using a new ticket each time.
Guards go around checking tickets at random times, and the fee for riding without a ticket is steep.
Watch the locals, and talk to fellow travelers for specific advice on how to manage the busses for the particular location. Coach travel is useful to get to towns that do not have train stations, or can be used almost exclusively. Again, whether the tickets should be bought beforehand or on the coach from the driver depends on the particular location. Luggage can usually be carried on board, but sometimes it is too full, and luggage is stored under the bus. I have been talking so far about coaches that run from town to town, and used by locals as well as travelers.