Molex is a maker of plug connectors. The 4-pin Molex plug is commonly referred to as a Molex plug.
The mainboard or motherboard is a computer's primary circuit board. Other hardware components (RAM, graphics cards, etc) are connected to the mainboard using plug-in slots. The mainboard is what brings the individual components together so that they function as a computer.
Mini ATX (µATX, ITX, Flex ATX)
The Mini-ATX form factor represents a miniaturisation of the ATX form factor. ATX mainboards are dimensioned to roughly 300 x 250 mm, while Mini ATX mainboards are only around 280 x 210 mm. The current smallest mainboard features the Pico ITX form factor and measures just 100x70mm. To fit these smaller designs, power supplies are also offered in small designs. Examples of this include SFX and TFX power supplies.