Southbridge is a micro architecture in a motherboard that connect : PCI/PCI-e/PCI-X, ISA bus/LPC Bridge (Though the ISA support is rarely utilized, it has interestingly managed to remain an integrated part of the modern southbridge. The LPC Bridge provides a data and control path to the SIO, the normal attachment for the keyboard, mouse, parallel port, serial port, IR port, and floppy controller; and BIOS ROM flash. ), SMBus (The SMBus is used to communicate with other devices on the motherboard, DMA controller (The DMA controller allows ISA or LPC devices direct access to main memory without needing help from the CPU), Interrupt controller (The interrupt controller provides a mechanism for attached devices to get attention from the CPU), IDE/ATAPI/SATA/PATA controller (interface allows direct attachment of system hard drives), Real Time Clock (The real time clock provides a persistent time account), Power management (APM and ACPI. The APM or ACPI functions provide methods and signaling to allow the computer to sleep or shut down to save power), Nonvolatile BIOS memory (The system CMOS, assisted by battery supplemental power, creates a limited non-volatile storage area for system configuration data).
Northbridge is a micro architecture that handle communication between processor, RAM, & graphic card.