What is a South Bridge?
A South Bridge is the Intel term for the main portion of the motherboard chipset which combines the interface between the processor and an assorment of other historically slower I/O peripheral devices such an integrated SCSI or IDE controllers, the keyboard controller, the PS/2 mouse controller, the real-time clock, and even serial ports.
However, it should be noted that a lot of the really slow I/O is off-loaded futher to a chip referred to as, "the Super I/O Chip." Depending on how tightly integrated the system is, the board may or may not contain this chip: remember that there are processors out there today with an integrated North Bridge, South Bridge, and so-called, "Super I/O," chips.
In your average system the South Bridge, the North Bridge, and perhaps some additional glue logic make up what is known as the motherboard's chipset.
On an Intel system, you can often identify the South Bridge controller as it is the one who's part number ends with the letters, "SB."