The southbridge is an IC on the motherboard responsible for the hard drive controller, I/O controller and integrated hardware. Integrated hardware can include the sound card and video card if on the motherboard, USB, PCI, ISA, IDE, BIOS, and Ethernet.
The southbridge gets its name for commonly being South of the PCI bus. Below is a graphic illustration of the ASUS P5AD2-E motherboard and some basic explanations of each of the major portions of the motherboard including the southbridge. As shown in the picture below, it is common for the northbridge and southbridge to have a heat sink; in addition, the northbridge is usually slightly larger than the southbridge.
Although the southbridge handles most of the I/O devices, less prominent input/output devices, such as a serial port, keyboard, and non-USB mouse are handled by the SIO (super input/output).
Note: Some newer chipsets are combining the Southbridge and Super I/O chips into a single chip and referring to this chip as the Super Southbridge chip. Some manufacturers such as NVIDIA and SiS have even combined the Northbridge, Southbridge, and Super I/O into a single chip.
Note: New motherboards are replacing the northbridge and the southbridge with IHA.