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, such as wires or traces on a circuit board, that allow different parts of the computer, like 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 can affect a computer's overall performance, as wider buses can transfer more data simultaneously.