The northbridge controls memory functions (less with AMD systems as they have an ondie memory controller on their chip), coordinates with the PCIe and AGP lanes and bridges the gap between the CPU and system memory (ram). Typically, this chip is located somewhere just below the CPU and usually has a large heatsink on it
The southbridge usually includes controls for the usb ports, firewire ports, serial and parallel ata, agp, raid and other component connections. It is usually located to the right of the various slots (pci, pcie, etc) and has a smaller heatsink on it