The southbridge is a chip that connects the northbridge to other components inside the computer, including hard drives, network connections, USB and Firewire devices, the system clock, and standard PCI cards. The southbridge sends and receives data from the CPU through the northbridge chip, which is connected directly to the computer's processor.
Since the southbridge is not connected directly to the CPU, it does not have to run as fast as the northbridge chip. However, it processes data from more components, so it must be able to multitask well. On Intel systems, the southbridge is also referred to as the I/O Controller Hub, since it controls the input and output devices.