A bus is a common wire connecting various points in a circuit; examples are the ground bus and power bus. The data bus carries digital information. A data bus is usually a group of parallel wires connecting different parts of a circuit with each individual wire carrying a different logic signal. The data bus is connected to the inputs of several gates and to the outputs of several gates. You cannot connect directly the outputs of normal gates. For this purpose three-state output logic is commonly used but will not be discussed here.
A data bus line may be time multiplexed to serve different functions at different times. At any time only one gate may drive information onto the bus line but several gates may receive it. In general, information may flow on the bus wires in both directions. This type of bus is referred to as a bidirectional data bus.