When we say diabetes , we actually mean Diabetes Mallitus in which there is dysregulation of blood sugar. But this diabetes Insipidus is a different clinical condition in which there is no blood sugar related issues.
It is a condition in which there is large volume very frequent urination and unquenched thirst despite drinking a lot of fluids. Even in mild form this condition is very trouble some as one has to wake up at night several times for urination. There is weakness of muscles, fatigue,loss of wt. ,dehydration ,electrolyte imbalance etc, etc.
This condition is cause by difficiency of a hormone called Anti Diuretic Hormone,responsible for re absorption of fluids in kidneys.
Please note that it is a very very brief summary about the disease.