PC builds are separated in many size categories. The components that mainly alter in each of these categories are the Case and the motherboard and in some cases the power supply and graphics card. The smallest form fact is the ITX which is recommended when you have limited space (it is not recommended for gaming rigs and overclocking since in a ITX build you are limited in terms of your cooling capabilities). Then there is the mATX which is somewhat bigger than the ITX, and it is a popular configuration since you can find cheap and quality components and can build a good gaming rig and not only. The next category is the ATX where the serious stuff come in. This is the most popular system size configuration in which most top quality, gaming and editing/designing, components are available. Once you jump into that category you can expect 4-digit and even 5-digit prices for a pc build since it is the top tier and all parts are basically available for you (since your case space capabilities do not limit you in any way). Finally, you have the E-ATX (Extended-ATX) motherboards which provide you with more layers of PCI-E and some more features in general but are not SO popular for budget related reasons.
I tried shortening as much as I could, so hope this helps!