The northbridge is a chip inside a computer that connects the central processing unit (CPU) to other primary components in the system. These components include RAM (a.k.a. system memory), the frontside bus (FSB), PCI Express cards, and the AGP card. The northbridge also connects to the southbridge, which controls the remaining components of the computer.
While the CPU is the main processor inside the computer, the northbridge is the primary controller. It acts like a traffic cop directing data to and from the CPU. Therefore, the performance of the northbridge chip affects the overall performance of the computer. On Intel systems, the northbridge is also called the Memory Controller Hub (MCH), since it controls the data flow to and from the system memory.