What is an peptic or duodenal ulcer?

What is an ulcer?  An ulcer is a sore in the mucosal (inner) lining of the stomach (called a peptic ulcer) or the upper small intestine (duodenal ulcer).  Most ulcer sores develop because of

1. taking non-steroidal drugs (NSAIDS) or

2. the presence of a bacteria called H. Pylori (helicobacter pylori) 

Both the NSAID drugs and the bacteria can damage the mucosal lining of the stomach/duodenum and a sore (ulcer) develops.