Sinuses are empty air spaces in the skull bones. They have openings to drain mucus from the sinus into the nose. People have 4 pairs of sinuses in the spaces.
The frontal sinuses near the eyebrows, the maxillary sinuses inside the cheekbones, the ethmoid sinuses between the eyes and the sphenoid sinuses behind the ethmoid sinuses.
Sinusitis is an infection of the sinuses in the face. Swelling and inflammation of the sinus mucosa blocking up the sinus passages to the nose. Obstruction can also be due to nasal polyps, a deviated septum or enlarged sinus turbinates.