The ATX or Advanced Technology Extended specification was designed and put forward by Intel in 1995. It was the first major change in the design of motherboards and computer cases 'since the older AT technology motherboards. ATX is now considered as the standard for motherboard forms. ATX addresses many of the issues faced by the AT form of motherboards that were seen as a problem for the manufacturers as well as the users.
The main difference between ATX and the previously used AT technologies lies in the power supply. The AT style computers had the power button connected directly to the system's power supply, meaning that you manually needed to use the power button to turn the system off. However, the ATX style computers don't have the power button connected directly to the power supply, allowing the user to turn the computer off directly through the operating system.
The older AT motherboards had two similar power connectors, which could be switched, causing irreparable damage to the motherboard. ATX motherboards have a single, keyed power connector which is impossible to connect wrongly.
Most ports like the 25 pin parallel port and the RS-232 serial port are being phased out in ATX motherboards, and being replaced with Ethernet, Firewire and eSATA ports.