Acute sinusitis is an inflammation of the sinus mucosa that is commonly caused by nasal infections, trauma, dental infections, or obstruction of sinus ventilation. It can affect the maxillary, frontal, ethmoidal, or sphenoidal sinuses. Common symptoms include headaches, pain and tenderness in the sinus areas, nasal discharge, and redness or swelling of the cheek. Treatment involves antibiotics, decongestants, steam inhalation, and analgesics administered medically or surgically through procedures like antral lavage or frontal sinus trephination.