To really understand what diabetes is, you need to start with the basic principles by which the body processes food into energy. Think about the foods you eat every day - most of them are converted to Glucose. Glucose is very important for those functions that the body must perform daily, and in essence it is a simple Sugar that is transformed from fuel into an energy source.
After eating, during the digestion phase, Glucose (blood Sugar) is distributed throughout the bloodstream to nourish the cells. At this point, blood Sugar levels rise, and under normal conditions, the pancreas produces a hormone called Insulin. The release of Insulin occurs in order to help Glucose penetrate into the body's cells.
The behavior of the body at this stage allows you to determine whether a person has diabetes or not. If a person has diabetes, he either has problems with the production of Insulin (in sufficient quantity), which helps Glucose to penetrate into the cells, or Insulin itself is enough, but it does not perform its functions.
A condition that prevents the body from fully utilizing the energy derived from food is the cause of diabetes, in the narrow sense of the term.