Alternatively referred to as the PAC (PCI/AGP Controller) and sometimes abbreviated as nb, the northbridge is an integrated circuit that is responsible for communications between the CPU interface, AGP, PCI, and the memory. Today, the northbridge is a single-chip that is North of the PCI bus, however, early computers may have had up to three separate chips that made up the northbridge.
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 picture below, it's common for the northbridge and southbridge to have a heat sink; in addition, the northbridge is usually slightly larger than the southbridge and is the closest to the CPU and 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.
My motherboard doesn't have a northbridge
As manufacturers look for new ways to improve performance and lower the overall cost they are moving the functions of the northbridge chip to other parts of the CPU and southbridge. Beginning with the AMD64 AMD and the Intel Nehalem processors the memory controller was moved from the northbridge to the processor die. Later, the Intel "Sandy Bridge" and the AMD Accelerated Processing Unit processors placed all northbridge functions onto the CPU.