The stomach is a large muscular organ in the abdomen that is part of the digestive tract. It takes food that has been mixed with saliva in the mouth and adds stomach acid to it to further denature and break down the food so that it can be digested. It also absorbs available glucose and water from the digesting food. Once food has reached the desired level of acidity, it is passed on to the duodenum and the small intestine for further digestion and the eventual excretion of waste products.
The stomach's lining is protected from stomach acid by a layer of mucous. However, some acid-loving bacteria can interfere with the mucous layer, allowing the stomach acid to start eating away at the stomach lining. This can lead to gastritis and ulcers. A bad enough ulcer can perforate the stomach and lead to peritonitis and sepsis, which are life threatening conditions.
If the stomach overproduces acid in reaction to the attempt to digest food, this can lead to it splashing back up into the esophagus. This causes the sensation known as heartburn. If heartburn becomes chronic due to constant overproduction of acid, the condition is termed acid reflux or gastro-esophagul reflux disease, a serious condition that can lead to long term damage.