Acid reflux also known as gastroesophageal reflux disease or GERD. It basically is a chronic situation with the mucosal lining of the esophagus. Symptoms are heart burn, difficulty swallowing, asthma, regurgitation, excessive salivation to name a few. http://en.wikipedia.org/wiki/Acid_Reflux#Signs_and_symptoms for more information.
Gastritis is simply inflammation of the stomach. Gastritis is caused by a bacteria that is found in the stomach lining called helicobacter pylori. Taking NSAIDS or aspirin, ibuprofen can help relieve the symptoms. http://www.gicare.com/Diseases/Gastritis.aspx