Control

Control units are in charge of the computer. Control units fetch and decode machine instructions. Control units may also control some external devices.
A bus is a set (group) of parallel lines that information (data, addresses, instructions, and other information) travels on inside a computer. Information travels on buses as a series of electrical pulses, each pulse representing a one bit or a zero bit (there are trinary, or three-state, buses, but they are rare). An internal bus is a bus inside the processor, moving data, addresses, instructions, and other information between registers and other internal components or units. An external bus is a bus outside of the processor (but inside the computer), moving data, addresses, and other information between major components (including cards) inside the computer. Some common kinds of buses are the system bus, a data bus, an address bus, a cache bus, a memory bus, and an I/O bus.