The southbridge is an integrated circuit on the motherboard that is responsible for the hard drive controller, I/O controller and integrated hardware such as sound card, video card if present on the motherboard, USB, 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 below picture, it's common for the northbridge and southbridge to have a heatsink; in addition, the northbridge is usually slightly larger than the southbridge.
Although the southbridge handles a lot of the I/O devices, slower and less prominent input/output devices, such as the serial port, keyboard, and non-USB mouse are handled by the super input/output (SIO).
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: Newer motherboards are also replacing the northbridge and the southbridge with IHA.