The sinuses are large cavities in the skull that sit behind the nose. They play a role in filtering and humidifying air drawn in through the nose and contain numerous mucous membranes. Like the brain, sinuses have numerous folds that allow them to have a very large surface area in a very small space.
However, the openings in the sinuses are very small - about the diameter of a pencil lead. As a result, they are very susceptible to being plugged by mucous or foreign material. When this occurs, the mucous cannot drain out of the sinuses, causing the feeling of a "stuffed nose" as well as being likely to cause a headache and creating a site for infection. Several common disorders, like the common cold and allergies can cause this.