TYPE 1 DIABETES, (in the past known as Insulin Dependent Diabetes, or Juvenile Diabetes) describes a type of diabetes where there is an absolute lack of insulin. The Beta cells (in the pancreas) normally responsible for making and secreting insulin die because the body's immune system has attacked them. Therefore no cells are available to make the insulin. In this case, the body will die without getting supplemental insulin in the immediate, or near-immediate future. The incidence of Type 1 diabetes has probably been underestimated in the past, and may represent 10% or more of people with diabetes. The only treatment at this time, is insulin shots.
In TYPE 2 DIABETES, (in the past, known as Non-Insulin Dependent Diabetes, Adult Onset Diabetes, Maturity Onset Diabetes, and others), beta cells are present, and therefore insulin is present. However the amount of insulin available is less than the individual requires (i.e., there is a relative lack of insulin). Furthermore, the insulin itself, in someone with Type 2 diabetes, doesn't work as well as it should when it gets to the cell, a state called Insulin Resistance. About 90% of diabetics fall into this category. Treatment may include proper diet, activity/exercise, and pills, but sometimes insulin is necessary.
There are several other causes of diabetes, but these are all secondary to something else going on in the body, such as pregnancy (gestational diabetes), damage to the pancreas (from any number of causes), or other hormone disorders.
Your doctor will tell you.
As a general rule, if you get diabetes at a younger age (younger than 25 or 30), you have no family history of diabetes or related diseases, and you're not overweight, you will likely prove to have Type 1 diabetes. If you are over 45, overweight and have someone in the family with diabetes (or an associated disorder, such as high blood pressure, heart attack, obesity, etc.), you probably have Type 2 diabetes.
These are generalizations, however, and there are clearly many exceptions to both.
If you are severely sick, with acids (what we call ketones) in your blood, you likely have Type 1 diabetes. If you don't have any ketones in your blood but it has a very high level of glucose, you may have Type 2 diabetes. Generally, if diabetes is diagnosed because a routine exam finds minimally elevated glucose, or it takes a Glucose Tolerance Test (see Diagnosis) to diagnose it, you probably have Type 2 diabetes.
The absolute level of glucose in your blood does not distinguish one type of diabetes from the other.