Diabetes mellitus (DM) is a group of diseases marked by high blood glucose levels due to insulin-related issues, affecting approximately 463 million adults as of 2019, with forecasts of 700 million by 2045. The main types include Type 1, Type 2, and gestational diabetes, each with distinct characteristics and causes, while complications can lead to severe health issues such as retinopathy and nephropathy. Management involves lifestyle changes, medications, and regular monitoring to maintain glucose levels within a target range.