Let us get things off with ATX, stands for advanced technology eXtended, a component compliance with the standard tells us that it will be physically and electrically compatible with other ATX components like computer cases and power supplies. Now become most desktop computers use ATX, which is what we are going to focus on, but there is other stuff out there anyway.
I will give you more details on each in a moment but if you are trying to figure out compatibility and that is all you care about, here it is. For power supplies, it comes down to the connectors on your particular power supply and the motherboard that you want use. Just make sure that the power supply has at least as many 24-pin, 8-pin, and 4-pin connectors as you need.
For cases on the other hand, any case that accepts a motherboard equal to or a larger than the motherboard that you are looking at will work. For example, a mini-ITX board will work in a case designed for any other standards here because they are all bigger than that. Whereas an ATX board will not fit in a case designed for micro ATX or mini-ITX but will work for the larger cases.
The exception here is XL-ATX, check specifically for XL-ATX compatibility before buying a case for your XL-ATX motherboard.
ATX motherboards have a wide range in cost but tend to be more expensive than mATX and less than eATX. They are 305 millimeters tall and 244 millimeters wide. They have seven expansion slots long for up to seven single slot expansion cards or up to four 2-slot expansion card if installed in case that has at least eight-tier expansion card openings and that makes ATX the minimum motherboard size to accommodate a three-way or 4-way graphics card configuration with modern cards.
ATX motherboards typically feature a 24-pin and an 8-pin power connector combination but performance-oriented board may have an extra CPU power connector or an auxiliary connector for configurations with lots of graphics cards. Because they are physically larger, they can have up to eight RAM slot and tend to have a more SATA ports and front panel header ports such USB, compared to micro ATX boards.
Micro ATX boards are less expensive and are 244 millimeter squared. They are available with nearly equal in performance and features to their bigger brothers with a couple of exceptions. Due to their size, micro ATX board usually only have two to four memory slots and can have a maximum of four single-slot expansion cards or two dual-slot expansion cards with a single-slot card in between. This means that while a multi-core Extreme Edition processor can be installed in a compatible mATX board, the maximum memory and graphics card capacity will be about half of an ATX equivalent. They typically feature a 24-pin connector and either a 4-pin or 8-pin CPU connector.
Mini-ITX motherboards are less expensive yet and are mere 170 millimeter squared. In recent years, they have gone from lower power quiet computing oriented products to high-performance small form factor gaming-capable devices. They are limited to desktop rather than workstation CPU platforms though and to a maximum of two memory slots and a maximum of 1 expansion card but then still house them at the time of filling this to have up to a quad-core CPU, 16 gigs of inexpensive RAM and a high performance dual-slot graphics card all the same time.
They have fewer SATA slots and front panel connectors than even mATX due to their small size but they often have integrated extras such as wireless and Bluetooth connectivity to compensate for their lack of expansion options. They usually have a 24-pin connector and a 4-pin connector for the CPU power.
On to improved things, extended or eATX motherboards are more expensive and larger than ATX. They reach up to 305 millimeters long by 244 millimeters wide. They are big. They still have only a maximum of seven rear expansion slots but they allowed them to accommodate 2 server or workstation class CPU's and up to twice as much RAM as their ATX equivalents.
Now because they are geared towards professional or server use, they tend to have fewer bells and whistles like better rear I/O and wireless connectivity but more robust connectivity internally with things like SAS ports replacing SATA ports that are more common on the desktop. They need a 24-pin connector and will usually have two 8-pins as well, sometimes more.
XL-ATX motherboards are better of an oddity. They do not even have a standard size but they are usually 244 millimeters wide and the size increase actually comes in the form of additional height. It is right in the name but it is more like it extra-large ATX motherboard than anything else.
Pricing wise they are similar to Premium ATX board. When it comes to performance, they have the same CPU, RAM, and rear slot configurations as ATX boards, which is weird. The extra height should allow them to have 8 or nine expansion slot but Manufacturers typically have used the extra space to give more room for large BP power delivery systems and cooling around the CPU socket rather than to add more expansion card slot, what a weird standard.
The one good thing that did come out of it is I think was that cases are now being delivered to market with more than seven slots at the back for better ventilation with multi-way graphics card configurations on even standard ATX boards. It is amazing to me that SL-ATX still exists.
