The motherboard or mainboard is the component that connects all the different parts of your computer. Often it has some integrated components like a sound card enabling your computer to play sound or a network card to connect your computer to the network. The standard form factor size for motherboards is ATX you need at least a midi-tower to fit one of those in. Micro-ATX motherboards are often more complete and have more onboard devices like a video board and fit in smaller cases but are generally less expandable.