Hypertension, also known as high blood pressure, is a condition where the blood vessels have persistently raised pressure. It is classified based on systolic and diastolic blood pressure readings into normal, prehypertension, and stages 1 and 2 hypertension. Essential or primary hypertension has no known cause, while secondary hypertension is caused by underlying conditions like kidney disease, diabetes or medications. Risk factors include family history, obesity, lack of exercise, too much salt or alcohol. Treatment involves lifestyle changes and medication to control blood pressure.