ATX was designed to address three major shortcomings of the earlier AT form factor, and it offers a few minor improvements as well. First, a designated portion of the board for the CPU socket keeps it out of the way of long cards, where AT boards had the CPU mounted behind or in place of card slots. Second, the inclusion of a port panel on the motherboard itself negates the need for slot brackets to break out such common items as USB, eSATA, or audio ports. Third, a cooling path from the lower front to the upper rear of the case vents hot air through the power supply and/or an exhaust fan. All three major improvements are centered on splitting the board between the slot and CPU area.
Most significant among the minor improvements was the addition of power-on through the motherboard. This allowed the system to turn itself off at shutdown and made possible such features as wake-on-ring (using a modem), wake-on-LAN (using a network adapter), timed power up/power down, and keyboard power-on hot-buttons.
ATX derivatives are based on the same CPU section, so that smaller boards are able to fit into larger cases if desired. ATX standards include microATX and FlexATX. Most Shuttle-style PC cubes (often called SFF for Shuttle Form Factor or Small Form Factor) use a two-slot variation of the FlexATX form factor reduced to approximately eight inches (later standardized by AMD as the exactly 8" DTX form factor), and VIA further shortened its mini-ITX form factor to 6.75" by reducing the maximum slot count to one. ATX size specifications are based on fractional inches.
The image above compares the maximum size and maximum number of slots allowed on various ATX-based form factors, with dashed lines indicating how the mounting holes in smaller boards still align with those of larger cases.