In the rotated pic of an Asus P5B Deluxe stolen, errr borrowed under fair use provisions for instructional purposes, from Newegg, the shinny square part in the middle that says Asus is the heatsink on top of the Northbridge. The copper colored bumpy square in the lower right hand corner above the S-ATA plugs is the heatsink on top of the Southbridge. They get their names from their location relative to each other (the top one is North and the bottom one is South) as they're mounted in an ATX tower case--which is why I rotated the pic.
Edit: Sorry, I forgot the function part of the question. Basically the Northbridge handles communication between the CPU and the RAM and the video cards and to the Southbridge. The Southbridge handles communication to everything else in the computer.