Motherboard form factors
Motherboards come in many shapes and sizes like ATX and micro ATX. ATX is the most common and fit into most cases. micro ATX is a scaled down version of ATX, so they normally have less PCI-E, Sata and ram slots.
The picture above is a ATX board created by MSI. The sockets on the top left are the only thing that will show ( unless your case has a window).
Case Form Factors
Cases come in many sizes and there is not actually a official form factor however they are manly split up into a mini tower, half towe, and full tower however there are lots of different sizes from stackable pc cases to flat desktops.
To run a server you need a massive system which you cannot fit in a normal case. in some massive servers you need entire rooms to contain the computer.
PC Case Sizes as Fast As Possible
Above is a server computer these can cost into the millions in hardware. however you will only need one if you run a massive server like steam or wikipedia.
CPU and RAM Form Factors
CPUs fit in certain type of sockets Each motherboard will only have one type of socket, However one socket does not just fit one cpu but a entire branch for example i3, i5 and i7 3rd gen processors fit in LGA1155 while 4th gen intel processors only fit in LGA 1150 sockets.
Ram form factors are very minimal, DIMM (desktop ram, more powerful then laptop) SO-DIMM (The more powerful laptop ram) Micro DIMM ( tiny ram with low power used for ultra thin laptops)
Hard drive and SSD form factors
Hard drives and SSDs only come in two form factors 2.5" or 3.5"