Diabetes is basically a disease that causes high blood sugar levels (which can be fatal if high enough) because your body is unable to produce or use insulin in order to properly regulate glucose in your body.
There are two types of diabetes, aptly defined as Type 1 and Type 2.
Type 1 diabetes is where the body’s immune system attacks the pancreas (which produces insulin) and destroys it. Because the pancreas no longer produces insulin, it needs to be administered through external means such as injection or an insulin pump. Proper diet and lifestyle changes makes it much easier to deal with blood sugar levels but a type 1 diabetic would still need to take insulin injections or use the pump for the rest of their life.
Type 2 diabetes is where the body either doesn’t produce enough or the body has developed a resistance to the insulin which interferes with the body’s ability to regulate glucose. This is by far the most common form of the disease and is most often linked to obesity/bad diet habits as the cause. People who have had type 2 diabetes in their family history are at higher risk of becoming type 2 diabetics themselves. For type 2 diabetics, it is possible to completely manage the disease through proper diet and lifestyle changes but it may required the assistance of oral medications and insulin.
In both cases, having diabetes doesn’t mean that you can’t eat sugar. It means that you have to be a bit more careful about your diet and lifestyle. By putting a little effort into learning about nutrition (particularly about glucose and carbohydrates), and cleaning up your diet, you dramatically improve your chances of living a long, healthy life.