Peptic refers to the stomach, and an ulcer is a sore or break in a membrane, so peptic ulcer disease describes having one or more sores in the stomach - called gastric ulcers - or duodenum - called duodenal ulcers- which are actually more common.
Normally, the inner wall of the entire gastrointestinal tract is lined with mucosa, which has three cell layers.
The innermost layer is the epithelial layer and it absorbs and secretes mucus and digestive enzymes.
The middle layer is the lamina propria and it contains blood and lymph vessels.
Then there’s the outermost layer which is the muscularis mucosa, and it’s a layer of smooth muscle that contracts and helps break down food.
In the stomach, there are four regions - the cardia, the fundus, the body, and the antrum.
So the epithelial layer in different parts of the stomach contains different proportions of gastric glands which secrete various substances.
Having said that, the cardia contains mostly foveolar cells that secrete mucus which is a mix of water and glycoproteins.
The fundus and the body have mostly parietal cells that secrete hydrochloric acid and chief cells that secrete pepsinogen, an enzyme that digests protein.
Finally, the antrum has mostly G cells that secrete gastrin in response to food entering the stomach. These G cells are also found in the duodenum and the pancreas, which is an accessory organ of the gastrointestinal tract.
Gastrin stimulates the parietal cells to secrete hydrochloric acid, and more broadly stimulates the growth of glands throughout the stomach.
In addition, the duodenum contains Brunner glands which secrete mucus rich in bicarbonate ions.
In fact, with all of the digestive enzymes and hydrochloric acid floating around, the stomach and duodenal mucosa would get digested if not for the mucus coating the walls and bicarbonate ions secreted by the duodenum which neutralizes the acid.