The sinuses are hollow structures in the skull located behind the forehead, nasal bones, cheeks, and eyes. They are filled with air and normally secrete small amounts of mucus that drains through the nose. Sinusitis is the inflammation of the inner lining of the sinuses. This generally occurs due to allergies or infections.
You don't often think about your sinuses until there's a problem. One day you realize you can't smell dinner cooking. Or you find you often have headaches or problems breathing through your nose.