Northbridge
Sometimes abbreviated as nb, the northbridge is an integrated circuit (generally Intel or VIA) that is responsible for the communications between the CPU interface, AGP, PCI and the memory. The northbridge gets its name for commonly being North 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 northbridge. 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 and is the closest to the CPU an memory.
When the CPU needs data from RAM, a request is sent to the northbridge memory controller. After the request has been received it responds with how long the processor will need to wait in order to read the memory over the front side bus (FSB). Some newer motherboards have replaced the northbridge and the southbridge with IHA.
Also see: Chipset, IHA, Motherboard definitions, Southbridge