Cholelithiasis is the medical term for GALL STONES. There are very common, and are caused by crystallization of cholesterol or bile salts/bile pigments in the gall bladder. If left untreated for an extended period time, they can cause a number of problems:
1) They can block the cystic duct: this means bile will not be able to be released into the small intestine, so fat from the diet can't be digested. It will lead to fatty, foul smelling stools, and probably pain.
2) They can leave the gall bladder, travel down the cystic duct and common bile duct and block the sphincter of Oddi: this means that neither bile nor enzymes from the pancreas will be able to enter the small intestine, which means neither fats nor most proteins from the diet can be digested.
3) They can be forced back along the pancreatic duct and into the pancreas, causing a backup of pancreatic enzymes: this is a very serious condition as the pancreatic enzymes will start acting on the pancreas itself and begin to digest it. It will cause swelling, extreme pain and inflammation (pancreatitis) which may eventually mean that it will stop working all together.
Gall stones can be easily treated with medications and/or ultrasound. The ultrasound is particularly useful if the stones have formed spontaneously (i.e. not through dietary deficiency or other underlying pathology) as it is non-invasive, cheap and completely safe.