Also known as Heartburn and Indegestion this is a pain caused by digestive acids going where they shouldn't and causing pain. Related conditions include Acid Reflux, Hiatus Hernia and Oesophagitis.
Causes are various, some are physical, ulcers can be responsible and bacteria are commonly implicated.
Heartburn - Royal College of General Practitioners Patient Information Sheet
Indegestion Peptic Ulcers, Heliobacter Pylori , Heartburn and Hiatus Hernia, Oesophagitis - Fact Sheets from Digestive Disorders Foundation
Hiatus Hernia Fact Sheet from PPP Healthcare