Located behind the bones of the face are hollow spaces filled with air. These hollow spaces are the sinuses and they act as a passage between the nasal cavity and the nostrils. So what is sinusitis? Sinusitis is an inflammation or swelling of the sinuses. Common symptoms include pain or pressure in the face, nasal obstruction or blockage, and a thick post-nasal drip. Those suffering from sinusitis may also experience headaches, a fever, or feel fatigued.