In ascending order, we would have; anorexic, skinny/average, chubby, fat, obese, and morbidly obese, in terms body weight. Chubby people often don't have anything to worry about, but fat people have to watch it or risk becoming obese.
I think obese is the only technical term. Fat and chubby are basically just other words for obese, or people can have their own opinions on the two words. Personally I think of "fat" and "chubby" as being used pretty freely. Anybody who is overweight is fat and/or chubby in my opinion.