a large vehicle carrying passengers along a fixed route with places for stopping on the way. London and many other British cities and towns still have some traditional double-deckers (= buses with two floors). Passengers get on and off these through the open door at the back, and a conductor walks around inside the bus selling tickets. The more modern buses have no conductor. Passengers pay the driver when they get on. The door is at the front, and only opens at official bus stops.
Buses used for long distances or tours are called coaches in Britain but buses in the US. Yellow school buses are a tradition in the US and parts of Britain.