Wikipedia > Chipset
http://en.wikipedia.org/wiki/Chipset
... the term chipset often refers to a specific pair of chips on the motherboard: the northbridge and the southbridge. The northbridge links the CPU to very high-speed devices, especially main memory and graphics controllers, and the southbridge connects to lower-speed peripheral buses (such as PCI or ISA). In many modern chipsets, the southbridge actually contains some on-chip integrated peripherals, such as Ethernet, USB, and audio devices. A chipset is usually designed to work with a specific family of microprocessors. Because it controls communications between the processor and external devices, the chipset plays a crucial role in determining system performance.

The manufacturer of a chipset often is independent from the manufacturer of the motherboard. Current manufacturers of chipsets for PC-compatible motherboards include NVIDIA, AMD, VIA Technologies, SiS, Intel and Broadcom. ...