Nowadays, there are dozens of different motherboard formats, each being tailored to one type of computer. However, we can identify the three most common models: the full-size form factor (ATX), the medium-size form factor (microATX), and the small form factor (mini-ITX). You can compare their main characteristics in the table below.
As you can imagine, the bigger the motherboard is, the more connectors and expansion possibilities it will offer. You should also bear in mind that the size of the motherboard determines the size of the PC case, thus affecting the cooling system you can mount.