A gallstone is a crystalline concretion formed within the gallbladder by accretion of bile components. These calculi are formed in the gallbladder, but may pass distally into other parts of the biliary tract such as the cystic duct, common bile duct, pancreatic duct, or the ampulla of Vater.
Presence of gallstones in the gallbladder may lead to acute cholecystitis, an inflammatory condition characterized by retention of bile in the gallbladder and often secondary infection by intestinal microorganisms, predominantly Escherichia coli and Bacteroides species. Presence of gallstones in other parts of the biliary tract can cause obstruction of the bile ducts, which can lead to serious conditions such as ascending cholangitis or pancreatitis.
Image from Lunar Caustic Flickr (source) Cholesterol stones in a gallbladder surgical specimen.
Gallstones may be asymptomatic, even for years. These gallstones are called “silent stones” and do not require treatment. Symptoms commonly begin to appear once the stones reach a certain size (>8 mm). A characteristic symptom of gallstones is a “gallstone attack”, in which a person may experience intense pain in the upper-right side of the abdomen, often accompanied by nausea and vomiting, that steadily increases for approximately 30 minutes to several hours. A patient may also experience referred pain between the shoulder blades or below the right shoulder. These symptoms may resemble those of a “kidney stone attack”. Often, attacks occur after a particularly fatty meal and almost always happen at night. Other symptoms include abdominal bloating, intolerance of fatty foods, belching, gas, and indigestion. (from wikipedia)