A conductor is a person who sells and checks tickets on a bus, tram or train. In the case of a train, he or she is also responsible for safely closing the doors and signalling to the driver that the train is ready for departure. It varies with the public transport system whether selling tickets is a regular procedure, or something that has to be done only occasionally, if a passenger has no ticket due to ignorance, carelessness, hurry or an attempt to have a free ride (fare-dodging). In those cases usually a higher price has to be paid. This can be presented as a fine on top of the ticket price or simply as a higher fare.
Checking tickets is sometimes done at the entrance of a station or at the entrance of a train, tram or bus, in which case people without ticket can not enter.
See also Routemaster, a bus in UK.