Diabetes mellitus is a chronic metabolic disorder characterized by high blood glucose levels due to either insufficient insulin production or resistance to insulin. The two main types are type 1 diabetes, where the body does not produce insulin, and type 2 diabetes, where the body does not produce enough insulin or cells are resistant to insulin. Symptoms include increased thirst, urination, appetite, and fatigue. Complications arise from long-term high blood glucose and include cardiovascular disease, nerve damage, kidney damage, and eye damage. Proper management through diet, exercise, medication and insulin therapy can help prevent complications.