Non-alcoholic fatty liver disease (NAFLD) is one of the types of fatty liver which occurs when fat is deposited (steatosis) in the liver due to causes other than excessive alcohol use. NAFLD is the most common liver disorder in developed countries.
NAFLD is related to diabetes and the metabolic syndrome and may respond to treatments originally developed for other metabolic-resistant states (e.g. diabetes mellitus type 2) such as weight loss, metformin, and thiazolidinediones. Up to 80% of obese people have the disease. Non-alcoholic steatohepatitis (NASH) is the most extreme form of NAFLD, and is regarded as a major cause of cirrhosis of the liver of unknown cause. Most people have a good outcome if the condition is caught in its early stages.
About 25% of people in the United States has NAFLD. With NAFLD, there are usually no symptoms. Some people may develop signs such as tiredness but fatty liver disease is usually a silent killer.
Fatty liver is a reversible condition wherein large vacuoles of triglyceride fat accumulate in liver cells via the process of steatosis (i.e., abnormal retention of lipids within a cell). Despite having multiple causes, fatty liver can be considered a single disease that occurs worldwide in those with excessive alcohol intake and the obese (with or without effects of insulin resistance).
A short John's Hopkin's video with additional information.