The human body wants blood glucose levels to remain at "normal levels." Once the glucose concentration in the blood increases, the body releases insulin which help lower the glucose levels. The process is explained HERE.
A bunch of cells called islets of Langerhans is a group of cells that produce insulin in the pancreas. In people with type 1 diabetes, these insulin-producing beta cells are destroyed or damaged, usually as a result of their own immune system (by an autoimmune process).