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ATX
The ATX form factor was created by Intel in 1995. It was the first big change in computer case and motherboard design in many years. ATX overtook AT completely as the default form factor for new systems. Some related designs include mini-ATX and micro-ATX. ATX addressed many of the annoyances with the AT form factor that system builders had to put up with. ATX will be replaced by BTX.
AT-style computer cases had a power button that was directly connected to the system power supply unit (PSU). The general configuration had four pins that had to be connected individually. Sometimes, the pins were soldered to the power button, making it difficult to replace the power supply if it failed, or if the fan inside the PSU seized up. The ATX version of the power supply didn't directly connect to the system power button, allowing the computer to be powered off via software. However, many ATX power supplies have a switch on the back to ensure no power is flowing to the motherboard (a trickle of energy is normally sent to an ATX-style motherboard even if the computer appears to be "off").
The power supply's connection to the motherboard was changed. Older AT power supplies had two similar-looking connectors that could sometimes inadvertently be plugged in incorrectly, generally causing short-circuits in the motherboard and causing it to fail. ATX used one large keyed connector instead, which made installation much easier and safer. However, the amount of power delivered fell below the requirements of newer AGP video cards and faster processors such as Intel's Pentium 4 and Advanced Micro Devices's Athlon, so some auxiliary power connectors were eventually added to the standard.
On the back of the system, some major changes were made. The AT standard only had a keyboard connector and little else. Other common data connectors such as serial and parallel ports had to be mounted individually. ATX allowed each motherboard manufacturer to put these ports in a rectangular area on the back of the system, with an arrangement they could define themselves (though a general pattern has been followed by most manufacturers). To avoid any gaps in the case, each manufacturer provided their own rear panel that could be inserted and fill any unused space.
The ATX form factor uses an 20-pin power connector located on the motherboard. The newest ATX specification uses a 24-pin power connector to support the power requirements of PCI Express.
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