The physical size of the motherboard.
The ATX board has at least 3 memory slots and at least 5 local slots PCI, which means that a well functioning powerful computer can be built on this board. However, a home or office computer does not need this high number of slots, therefore also microATX will be suitable.
The connector of the power supply unit of the motherboard is the power source socket which are currently of ATX and ATX2.0 standard. ATX is an older standard with a 20pin connector (20 "pins"), and ATX2.0 has 4 additional pins (20+4). The motherboards of ATX standard can operate with a ATX2.0 power supply unit (PSU), however the mother board of ATX2.0 standard requires an ATX2.0 power supply unit.
Form-factor is rather a recommendation. However, prior to buying a mother board you should verify the compatibility of the housing size to the motherboard size and its power supply connector.