The motherboard is the largest and most fundamental component of a PC. Every other hardware component is somehow attached to the motherboard. The motherboard is the common link for every component to communicate and work together.
The motherboard has a series of slots, sockets and connectors for connecting the various components of a PC. The memory, accessory cards, and CPU are installed directly onto the motherboard in most cases. The drives and peripherals communicate with the motherboard through wired connections.
It is becoming increasingly common for motherboards to integrate features that used to require separate accessory cards. Most motherboards integrate drive controllers and communication ports; and with greater frequency they integrate sound, video and network features as well.
There are a wide range of motherboards to choose from. They differ in features, speed, capacity and the CPU supported. They also differ in size, shape and layout, this is commonly referred to as the "form factor".