A bit of research into the specific features of your motherboard can help you decide what to buy, and it can protect your computer from damage from hardware which is not compatible with your motherboard. The most common styles of motherboard are the AT form factor motherboard, along with its next-generation form factor cousin, ATX.
AT stands for "Advanced Technology." ATX stands for "Advanced Technology Extended." AT and ATX are form factors of motherboard. "The Complete Guide to A+ Certification" by Michael Graves defines form factor as "a term that defines the physical layout of a specific component." Form factor also denotes the type of hardware and power you can connect to your hardware component. Form factor is not just a term for motherboards.
Size and Orientation
The ATX form factor's positioning was redesigned to offer better access to the peripheral components on the inside of the computer. Both AT and ATX motherboards have been produced in various sizes throughout the years, and the form factors fit in different computer cases depending on their size. An ATX motherboard is positioned at a 90 degree angle from the positioning of AT motherboards.
A notable difference between the ATX motherboard and the AT motherboard is the addition of "sleep" mode to the ATX form factor. Sleep mode is a power management mode in which some of the components are powered down to save power, but parts of the computer remain ready to boot. The sleep mode reduces the use of power when the computer is not in use, while still allowing you to more quickly revive the computer and return to where you left off. In addition, the power supply in the ATX motherboard more easily converts 5 volt current to 3.3 volts when necessary, and involves less circuitry in the process of converting the power.
The power connectors also differ between AT and ATX motherboards. AT motherboards use two 12-pin plugs to power the motherboard. An ATX motherboard uses one 20-pin plug for the power supply. When using an ATX form factor motherboard, you must use an ATX power supply. You can use the pin number to identify whether you have the correct power supply for your motherboard.
The outside connectors on the AT and ATX motherboards are the most visibly noticeable difference between the two form factors. The AT form factor motherboard is limited to one outside connector, a five-pin DIN connector for the keyboard. An ATX-style motherboard is built to incorporate many other connectors, including connections for network cards, video cards, sound cards and modems.