The power connector is different, and ATX introduced the panel on the back with the PS/2 keyboard/mouse connectors, usually also where you find network, video, serial, parallel, etc. AT just had the old school AT keyboard plug and everything else required a separate connector from the motherboard or a PCI/ISA card.

Also, ATX supports the shutting down of your computer through the software... AT, however, would end at the screen that read "It is now safe to turn off your computer" and you manually had to power it off.

All Motherboards used to have the same rear connector layout for the keyboard and the same width from the days of the 286 into the early Pentium days. This is where the term, AT, came from. It was the form factor of the IBM AT PC Motherboard, which was very different from the XT systems. All of the Motherboards had a round, 5 pin DIN connector for the keyboard about 3/8" in diameter. The keyboard connector was part of the Motherboard. It lined up with a hole in the rear of the PC's case. The serial and parallel ports came from cables from an add on card. These mounted on the rear slots or knock outs on the rear. Later these ports were on the Motherboard with the IDE and floppy controllers, but they still had cables connecting the ports to the rear of the PC case. When the on board PS/2 mouse port became common, it just was one more cable from the Motherboard to the rear panel. A typical AT type case and Motherboard are shown below. The cards starting from the far right to left are a modem, sound card, and finally a video adapter. AT Case and Motherboard end views. The Keyboard connector is on the left of the Motherboard. It lines up with the hole in the case above. AT MB bottom half top view. This MB has built in IDE, floppy, serial, parallel, PS/2 mouse, and USB ports, but requires a ribbon cable for all of them. These connectors really started to get in the way of other cards. The extra ribbons running inside the box were bad enough, but the connectors themselves often covered up the end slots so that you couldn't put in a card where you wanted to. Sometimes these I/O connectors wouldn't fit because the hit parts on the MB. The first attempts to reduce case clutter was to make a hybrid type MB/case. The KB and mouse connectors were the small, PS/2 connectors. The manufacturers put them in the same location as the the older AT KB connector, so the same cases could be used with little modifications. In fact most hybrid cases were the same as the AT case. The only change was a very small metal shield to cover the size difference between the PS/2 connectors and the larger, 5 pin DIN. Micron Technology and Gateway made a lot of these in the late 486 and early P90 days. These MBs were all PCI and had on board I/O. They still used the same AT power supply and footprint. As more cables and cards filled the box, cooling became a problem, too. The AT power supply sucked air from the rest of the box and pulled it through the power supply to the outside world. The only parts that got the benefit of high speed air were those in the power supply. Everything else used slow moving air. This air wasn't even cool, since it dragged across hot parts as it moved towards the power supply. For busy PCs extra cooling fans or fan cards had to be added to prevent hot spots in the case. CPUs kept generating more heat as the fast MMX chips came around, and the PIIs offered even more cooling problems. Something had to be done. Enter the ATX Motherboard. The ATX case and Motherboard uses a large fan in the power supply that sucked air in and forced it right over the CPU and heart of the Motherboard. This solved the CPU and Motherboard cooling problems. Early ATX power supplies had the fan external to the power supply which hit the PII chips and regulator cards for PPro Motherboards. Not good! This problem has been fixed. All PII systems are ATX form factor because of the cooling advantages. ( and because Intel developed ATX just for that reason, and they make the PII chips!) An added bonus is that the power supply has no live 120VAC anywhere in the case. You can't work on a PC and cook a part or yourself by a slip of a metal screwdriver. Power ON/OFF is via a small, logic level sensing circuit on the Motherboard and a momentary contact switch on the front of the case. The nicest feature is that the serial, parallel, keyboard, mouse, and now USB ports are all in a cableless cluster on the Motherboard. These mount via a rectangular shield inserted into the case. The keyboard and mouse are PS/2 connectors now. They are the two connectors stacked on the left side of the photo below. The USB slots are next to them to the right. The good news: Hey, no mess! The bad news: ATX Motherboards don't fit in an old AT case. They are wider and require a rear mount cutout in the case. The power supply has the same mounting pattern, but it is different, too. For new systems ATX is the way to go. It only costs $10 more than AT in the Pentium class machine, but offers better cooling, a cleaner case, and a case that will support a PPro or PII in the future. For upgrades, the Motherboard will almost always be an AT style. ATX connector cluster with USB If the Motherboard has a built in audio card, which a lot of the PII systems do have, the audio connectors and the joystick port would also reside on the ATX output plate. On this photo they would be on the far right. A note on AT and ATX power supplies: AT and ATX power supplies are not interchangeable in general. Although functionality is basically the same, the control wiring and wiring harnesses are completely different. Some of the newer AT Motherboards have both types of power connectors. However, the case for the two types of Motherboards are usually different and will only hold one type of power supply. A few ATX cases now exist with the AT or ATX rear shield plate, so you could use an AT Motherboard if it had both power connectors. These are rare. The big electrical difference between the two is the ATX supply uses logic on the Motherboard to provide a momentary ON or OFF pulse via a DC control line. All line power lives inside the power supply's case. This is a good, safe improvement over the AT supply which had an AC line cable to a switch on the front panel. Besides the safety issue, having that extra cable snaked through the case to the front panel made service harder. Low cost manufacturing often dictated soldered the cable to the switch. Now half the case had to be disassembled to change the power supply. If the PC case manufacturer used a non standard, cheap switch, which happened a lot, you would be forced to solder the new supply onto the old switch. So the next power supply service call would be just as nasty as the last one. To test both types of power supplies they need to be connected to the Motherboard. All power supplies have feedback wiring to make sure power doesn't come on to a system with a short circuit. It is a logicical AND function of the -12, +12, and +5V buses. If all voltages are within range, the logic line to the power supply goes high and the supply stays on. This practice started in the 286 and 386 days when a CPU or math coprocessor installed incorrectly could start sending smoke signals in your case. Early AT supplies would still turn on with the load of a HD, but all the later ones need the load of the Motherboard for the logic line to be set correctly. If you ever here your power supply making a really low pitched buzzing sound, and everything seems quite dead, chances are your PC has a short in it. The most common short is the Motherboard's underside touches the case. You lost an insulating washer when you last replaced the Motherboard. Or you put in a new Motherboard, but left the old standoffs in place. One of them doesn't line up with a hole on the new Motherboard but does line up with a few solder bumps

Wow, those are some serious answers. Mine's a little simpler: AT: 80286 to 80486DX4-100 Bus Slots: PCI, VESA, EISA, ISA and 8-bit RAM: SIMM, SIP ATX: Pentium I and newer, Bus Slots: PCI, AGP and PCI-Express RAM: DIMM