These occur when two cats have been fighting. They are often seen around the face and tail base and are the result of a bite wound becoming infected.
Initially your cat may seem a bit quieter than normal and you may notice them over-grooming a particular area which will appear swollen. Once the abscess has burst your cat will most likely feel less painful but may have a smelly open wound instead.
This is usually easily treated by lancing the abscess, cleaning and flushing it out and treatment with appropriate antibiotics.