Diabetes mellitus is a metabolic disorder marked by impaired glucose utilization, leading to complications in fat, protein metabolism, and electrolyte balance, primarily due to insulin deficiency or resistance. The document outlines various types of diabetes, including Type 1, Type 2, gestational diabetes, and impaired glucose tolerance, emphasizing the need for insulin management, dietary adjustments, and lifestyle changes for effective glucose control. Symptoms include excessive thirst and urination, increased appetite, and complications can arise from poorly managed blood sugar levels.