Tonsillitis is inflammation of the tonsils (tissue at the back of the throat) usually caused by viruses or bacteria, leading to symptoms like sore throat, fever, difficulty swallowing, red/swollen tonsils (often with white spots), and swollen neck glands, commonly seen in children but also affecting teens and adults; treatment depends on the cause, often resolving with rest for viral cases or antibiotics for bacterial ones like strep throat