When you eat, your body breaks carbohydrates down into glucose. At that point, your pancreas is supposed to release insulin.
Insulin is a hormone that is supposed to help glucose enter your cells so your body can use the glucose for energy.
In people with diabetes, this system does not work—either because the body cannot make enough insulin (type 1) or it does not use the insulin it does create (type 2). There are other forms of diabetes, including gestational diabetes, which occurs during pregnancy.