As stated earlier, the microprocessor works on the data stored in RAM. Further, to execute the instructions stored in ROM, it has to fetch these instructions into RAM. Communication between the microprocessor and the memory chips as well as other chips found on the motherboard is accomplished through a set of wires running between them. These are called the bus. The bus comprising a set of 8 wires for carrying data is called the data bus. When data is sent from one unit to another, the address (location in memory at which the data is stored) is also sent with the data.
The bus has a set of 20 wires (in an 8088-based system) for carrying these addresses. This type of bus is called the address bus.