The motherboard is the main circuit board that connects all the parts of a computer system. It contains important components like the processor socket, RAM slots, expansion slots, and connectors. These include the CPU socket to install the processor, RAM slots to connect memory, PCI and AGP slots for graphics and other add-on cards, and connectors for devices like hard drives, floppy drives, and ports. The motherboard allows all the components of the computer to communicate with each other so they can function as a unified system.