Proton pump inhibitors (PPIs) are a class of medications that reduce stomach acid production by blocking the enzyme responsible for acid secretion in the stomach lining. They are commonly used to treat conditions like gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome, but long-term use can lead to potential side effects such as nutrient malabsorption and increased risk of infections.