What is the main difference between diabetes mellitus and diabetes?

Nothing: Usually when someone says diabetes, they are referring to diabetes mellitus. There is a condition called diabetes insipidus but that is a completely different disease involving sodium and water concentrations in the blood and urine.

Nothing: I'd say 95+% of the time we say "diabetes" we are speaking of diabetes mellitus, a condition characterized by elevated blood sugar and associated metabolic issues. There is a condition called diabetes insipidus, but it is relatively uncommon.