A computer bus is a communication system that transfers data between components inside a computer or between computers. It consists of a set of physical connections, like wires or traces on a circuit board, that allow different parts of the computer, such as the CPU, memory, and input/output devices, to communicate with each other. Buses can be classified into different types, including data buses, which carry actual data, and address buses, which carry information about where the data should go. The speed and width of a bus, measured in bits, determine how much data can be transferred at once, impacting overall system performance.