Most cases mount one or more case fans, distinct from the fans that may be
attached to the power supply, video card and CPU. The purpose of a case
mounted fan is to move air through the system and carry excess heat out.
This is why some cases may have two or more fans mounted in a push-pull
configuration (one fan pulls cool outside air in, the other pushes hot interior air
out). The more air these fans can move, the cooler things will generally be.
Fans currently come in two basic sizes, 80mm and 120mm. Cases tend to
support either one or the other; in that the case fan mounts can hold that size.
The larger 120mm fans spin more slowly while moving a given volume of air,
and slower fans are usually quieter fans, so the 120’s are generally preferred
even though they cost a little more. Good 80mm fans can still be pretty quiet
though, so while fan size is a factor, it shouldnt be a deal-breaker if the case
has other features you like.
Make sure the power plug on any case fans is supported by your mother
board, 3 and 4-pin Molex connectors are common. Fans can also be powered
directly by the PSU, but in that configuration, the motherboard cant control
or report the fans speed.