A bus is simply a length of cable which carries power or data, it may consist of 1 or more wires, commonly two.

Common bus types found on model railways are:

Power bus: Carries DCC power and data around the layout
Accessory Bus: Provides power and optionally data for accessory decoders, commonly uses a separate power supply to the main power bus.
Data bus: Connects items together and carries data, a common use is in providing a data link for boosters or connecting block detection devices.