Encephalitis is inflammation of the brain, whereas meningitis is inflammation of the lining of the brain. Fibrositis is a painful muscular condition due to inflammation of the tissues around the muscles themselves, sometimes with the formation of painful nodules.
There is also the 'itis' of joints, arthritis. However, arthralgia just means pain in a joint, just as neuralgia means nerve pain or pain along the course of one particular nerve. Hence, we often speak of neuralgia when we have toothache.
These are just some of the common conditions I see in the medical world - I could go on forever with the list of jargon used for illnesses. Instead, the most important thing to remember is not to be frightened by jargon as, more often than not, it sounds more serious than it really is.
Many articles taken from 'A word with the doctor', by Dr. John Windsor.