The motherboard is the computer's main circuit board. It's a thin plate that holds the CPU, memory, connectors for the hard drive and optical drives, expansion cards to control the video and audio, and connections to your computer's ports (such as USB ports). The motherboard connects directly or indirectly to every part of the computer.
This motherboard was developed by intel in 1995. ATX is the most common motherboard design. Several ATX-derived designs have been specified that use the same power supply.
ATX (Advanced Technology Extended): ATX is a full size board measuring 12" wide by 9.6" deep. ATX has 6-pin mini keyboard connector. Also, it has double row single power supply connector providing +/-5V, +/-12V, and +3.3V.
MicroATX: MicroATX is a small motherboard size of 9.6" x 9.6". Compared to full size ATX, microATX have smaller number of I/O slots. For example, full ATX can have 5 PCI slots, whereas MicroATX can have up to 3/4 PCI stots only. The chief advantages over ATX is reduced size, and power requirements. The microATX form factor was developed as a natural evolution of the ATX form factor to address new market trends and PC technologies. The microATX form factor improves upon the previous specification in several key areas.
ITX: ITX is the smaller version of the motherboard. The user will find only a few expansion slots that are because of its small size. Also, one can find the ITX mini as well which have only one expansion slot. Since the space on this motherboard is less, so one might find very less features on this motherboard. The mounting points which would be found by the user are same as the ones he would find in Micro ATX or the ATX. So, one can fit them one by one in the same type of case. So, many manufactures actually build the case in the way that whatever motherboard is used, it can get fit into the case. SO basically, the Mini ITX gives an edge to the user if he wants to have the motherboard which covers only a small room.
Mini-ATX: Mini-ATX motherboards were designed with Mobile on Desktop Technology which adapt mobile CPUs for lower power requirements and less heat generation. Mini-ATX motherboards use surface-mount technology and solid state capacitors.