Fans

The case of the computer needs a place for case fan(s). A case will always have at least one fan but can have many more fans. 

Case fans are different than CPU fans (CPU section) they are much larger and provide airflow for the whole case. You want as much air circulation in your case as possible. The problem is that the more fans you have the louder it gets, so remember your threshold to sound. Also, remember that the size and RPM of the fan make a difference. If you want quiet use larger fans with more surface area and less RPM.  The animation below visually shows this concept.