There are an absolutely crucial component of modern mother boards (and/or graphics cards .ect). Basically, they control the flow of bits to/from peripherals, memory, and the CPU.
See the link below for a simple diagram of the control flow through an Intel chipset. The two blue squares are the North bridge and South Bridge chips that make up the "chip
set":
http://developer.intel.com/design/ch...chpst+diag_lg&