Stands for Advanced
Technology Extended (PC). Refers to the computer case,
power supply and
motherboard specifications. The older variety is AT.
When you assemble a machine, the 3 components mentioned above must be all of the same kind, either AT or ATX. The ATX power supply additionally offers a 3.3V power and 5V stand-by for keyboard power-on, Wake-On-LAN and possibly a non-interrupted USB slot. It is also software-driven, while the older AT kind can be only turned on/off manually.