Computer bus

A computer bus (often simply called bus) is part of most computers. Its role is to transfer data, signals, or power between some of the components that make up a computer. 

The size or width of a bus is how many bits it carries in parallel. Common bus sizes are: 4 bits, 8 bits, 12 bits, 16 bits, 24 bits, 32 bits, 64 bits, 80 bits, 96 bits, and 128 bits.

Computers use such buses to link:
 CPU to on-board memory;
 multiple CPUs in a multi-CPU system;
 arithmetic logic unit to the rest of the CPU;
 hard drives, graphics cards, etc. to the main system.

Drives are usually connected by Serial ATA or the earlier ATAPI bus. USB and Firewire connections go to printers and other peripheral devices.