Cholecystitis refers to inflammation of the gallbladder. It is most commonly caused by gallstones which can block the cystic duct and cause bile to build up. There are two main types - acute cholecystitis which occurs suddenly and causes severe pain, and chronic cholecystitis which is a long-term lower intensity inflammation. Diagnosis involves blood tests, imaging like ultrasound or CT scans. Treatment options include pain medication, antibiotics if infected, and surgical removal of the gallbladder (cholecystectomy) for severe or long-term cases.