From toms-hardware.ru.
The actual function of a chipset is to communicate between all components of the modern PC, or, to put it in other words, the chipset is the infrastructure. The chipset contains numerous components, and moreover, it makes various interfaces available for connecting additional components (PCI, USB, AGP, IDE, etc). As a rule, the chipset consists of two units, the Northbridge and the Southbridge. The reason that it is made of two units instead of one is that it's difficult to integrate all components onto a single chip. The use of two chips also allows for different combinations of Southbridge and Northbridge chips. Today, most manufacturers have a small building-block type of system from which the motherboard makers can choose components to suit customer requirements.
The Northbridge usually contains the CPU interface and the memory controller. With nForce and nForce2, the graphics unit is also on the Northbridge. This chip is clearly larger in size compared to the Southbridge, because hundreds of data paths are needed for the processor and the RAM. Because of this, all complex chips are packed in BGA (Ball Grid Arrays) casing, where all the soldered connections are located on the underside of the chip.
For years, the PCI bus was used as the connection to the Southbridge. However, its bandwidth is no longer sufficient for today's requirements, so the chip makers all offer their own solutions (e.g., VIA's V-Link, SiS's MuLTIOL, Intel's i-Link and AMD/NVIDIA's HyperTransport).
Currently, the Southbridge contains at least a PCI controller, floppy/ IDE/ hard disk controllers, serial and parallel ports, USB support and power management functions. Through the years, the chipsets have gained extended functionality through the Southbridge, which is the reason that nowadays these units often also include interfaces and codecs for sound or network controllers. This means that motherboard manufacturers only need to provide simple controllers, which are much cheaper to produce than complete PCI network cards or sound cards.