Those little pathways that channel data and instructions between memory, registers and elements (or as jakohn said it, multiple devices).

The word bus is derived from the latin omnibus with the idea that several sets of chips can be connected to the same bus and share the same communication channels.

Three buses are usually within the processor, the data bus (for transferring data, duh), the address bus (for communications between the control unit and memory) and the control bus (for transmitting signals between components).