A MotherBoard - A motherboard is the brain of the computer and is what you build everything onto - the base of your mining rig. The main feature you are looking for in a motherboard is the number of GPU slots it has as this will determine how many graphics cards or GPU's it can fit - and in the end your total hashing power. 3 PCI Express slots will mean you can fit 3 x Radeon HD 7950 with a hashrate of 20 MH/s each - or a total hashing power of 60 MH/s. A PCI Express slot is a connection port on the motherboard and looks like the picture below - they are often coloured white but can be beige - you can get other types of slots but most GPU's work on a PCI express.
We used the GA-Z97X-Gaming 3 which you can get here
RE: For those thinking of getting into Ether mining read this