Diabetes, also known as diabetes mellitus, has several forms, but each is characterized by excessively high blood glucose or hyperglycaemia.  Hyperglycaemia is caused by either defects in insulin production or insensitivity to insulin, or both. Other causes are autoimmune or destruction of  beta cells in the Islets of Langerhans of the pancreas which affects insulin production and impaired action of insulin on target tissues.
There are two main types of diabetes, namely the Type 1 Diabetes (formerly known as Insulin-Dependent Diabetes Mellitus) and are commonly diagnosed among children and young adults,  and the Type 2 Diabetes (formerly known as Non-Insulin Dependent Diabetes Mellitus). There are also other specific forms of diabetes namely:

• Gestational Diabetes Mellitus (common among pregnant women)
• MODY(Maturity Onset of Diabetes of the Young
• Genetic Defects in Insulin Action such as Leprechaunism and Rabson-
Mendehall Syndrome
• Diseases of the Exocrine Pancreas such as Pancreatitism Cystic Fibrosis
and Haemochromatosis
• Diabetes that develops as a feature of Endocrinopathies
• Diabetes caused by drugs or chemicals such as  Corticosteroidsm Thiazide
Diuretics and Beta Blockers
• Diabetes caused by Infections such as Rubella or Congenital German Measles
• Uncommon forms of Immune-Mediated Diabetes such as  Stiff Man Syndrome
• Other genetic syndromes which may confer a greater risk of Diabetes such
as Neonatal Diabetes and Mitochondrial Syndromes passed through the
 maternal line, Down's Syndrome, Turner's syndrome, Klinefelter's Syndrome,
and Friedrich Ataxia.

In Type 1 Diabetes, the body does not produce insulin. Insulin is a hormone that is needed to convert sugar (glucose), starches and other food into energy needed for daily life. There is a severe to total insulin deficiency which results in a decrease in glucose uptake by the muscle cells, breakdown of muscle into amino acids – then taken up by the liver and converted into new glucose(gluconeogenesis), an increase in the breakdown of triglycerides, leading to an increase in the release of free fatty acids – also taken up by the liver and converted into a usable energy form, ketones, and a reduction in the uptake of glucose by the liver and an increase in the breakdown of glycogen (glycogenolysis).
Incidence of type 1 diabetes is now increasing. There are studies which shows  geographic variation prevalence of type 1 diabetes. According to International Federation of Diabetes, Finland reports the highest incidence (cases per 100 000 people per year) of type 1 diabetes (45.0) and Singapore one of the lowest (2.0). Type 1 diabetes occurs in all populations and across the socio-economic spectrum. However, there is some evidence of an association with groups of higher socio-economic level. This may reflect the inability of people from lower socio-economic classes to access appropriate medical care. In countries and communities where insulin is not available or inaccessible due to cost, many people with type 1 diabetes die before diagnosis.

Type 2 Diabetes is the most common type of diabetes, usually 90%-95%. Unlike in Type 1 diabetes, people with type 2 diabetes still have some insulin production. Indeed, they may have higher blood levels of insulin than their counterparts without diabetes – although  in response to higher-than-usual blood glucose concentrations. However, type 2 diabetes does not occur without beta-cell destruction. Excess body fat – particularly abdominal or visceral adiposity – is thought to contribute to the insulin insensitivity that is characteristic of this type of diabetes. It has been shown that people who carry excess weight around their abdomen are at higher risk of diabetes. Type 2 diabetes tends to develop slowly such that the symptoms of this condition, and therefore, the diagnosis, are often missed. Longstanding undetected hyperglycaemia – as often precedes the diagnosis of type 2 diabetes – may lead to the development of long-term diabetes complications. Complications are therefore often present at the time of diagnosis.