ATX and other Form-Factors
There are several different types of cases (form factors) available for your custom PC build, but I recommend getting one of the 2 following case types as they are the most common and most widely compatible with just about all PC components.
ATX
ATX cases are by far the most common cases used for custom PC builds and are also the most compatible with just about any motherboard you would want to buy (as well as offer the widest compatibility range for almost all pc mods and cooling accessories). For just about any rig, I would recommend buying an ATX case over anything else for these reasons. ATX cases are the typical mid or full-tower cases that you can find at most PC hardware retailers. Here are a few examples:
Full-Tower
Micro-ATX
Micro-ATX cases are popular with some due to their conveniently smaller size. They are less common than ATX cases, and by and large there aren't as many motherboard models that are sold in a Micro-ATX form factor, so going with this case will limit the motherboard options you will have to use for your PC. However, if you want a smaller case, and find a Micro-ATX motherboard that you want to buy that meets your system needs, then by all means, buy it! This is your system and if a Micro-ATX is more your style, GET IT! Here's a few examples of what Micro-ATX cases look like:
