The hiatus is an opening in the diaphragm which is the muscular wall separating the chest cavity from the abdomen. Normally, the esophagus (food pipe) goes through the hiatus into the stomach. In a hiatal hernia, the stomach bulges up into the chest through that opening.
Hiatal hernias are classified into two main types. A sliding hiatal hernia is when the junction of the stomach and the esophagus herniate and slide up into the chest through the hiatus. This is the most common type of hiatal hernia. A paraesophageal hernia is less common, but is more cause for concern. The esophagus and stomach stay in their normal locations, but part of the stomach squeezes through the hiatus, placing it next to the esophagus. Although you can have this type of hernia without any symptoms, the danger is that the stomach can become "strangled," which means its blood supply is cut off.
Often, people with a hiatal hernia also have heartburn or gastroesophageal reflux disease (GERD). Although there is a link, one condition does not necessarily cause the other, because some people can have a hiatal hernia without having GERD, and vice versa.