Motherboard is the heart of all PCs. Every components are sitting on the motherboard.
Motherboard controls the Voltage and all others through out the components.
There are a lot of motherboard types out there today but the most common ones are BTX and ATX.
BTX was vanished in the mid 2006 and now, the ATX type motherboard is common in the market.
BTX was originally made for better air cooling (cooling efficiency). The BTX case is specifically designed for the BTX motherboard. The air comes from front and leave through the back of the case.