What are diabetes insipidus and diabetes mellitus?

Diabetes insipidus is simple failure to concentrate urine, because of a lack of (or resistance to) the anti-diuretic hormone Vasopressin (aka "ADH"), made in the hypothalamus and stored in the pituitary gland in the brain.

The name "Diabetes insipidus" comes from the victim's copious urine ("Diabetes" means 'funnel' or 'spout') and the tasteless nature of the urine, which is mostly water. (it's insipid).

Diabetes Mellitus refers to "sweet" (mellitus) copious urine. Again the victim urinates copiously, but this time the urine is sweet.

This is because a DM sufferer is short on a different hormone, insulin, produced by the beta cells of the pancreas. This hormone's job is to regulate energy use by allowing the body to use glucose, its chief fuel for energy. If the hormone is absent or ignored, the body refuses to accept glucose from the blood. The glucose builds up in the blood, eventually being filtered out by the kidneys, where it causes the production of lots of extra-sweet urine.

Both conditions cause polyuria (excess urination), polydipsia (excess thirst/drinking), and dehydration, but for very different reasons. This is why doctors used to need to taste the urine to give a diagnosis.