Primary or gastric causes of vomiting are those that are due to diseases of the stomach and upper intestinal tract. Secondary or non-gastric causes of vomiting are caused by diseases of other organs that cause an accumulation  of  toxic substances in the blood. These toxic substances stimulate the vomiting center in the brain causing the animal to vomit.

A problem that can be confused with vomiting is regurgitation. Vomiting is the ejection of contents of the stomach and upper intestine; regurgitation is the ejection of contents of the esophagus. The esophagus is a narrow, muscular tube that food passes through on its way to the stomach. In health, food moves quickly through the esophagus to the stomach. If the muscle of the esophagus loses tone, the esophagus dilates, a condition called megaesophagus. A dilated esophagus does not effectively move food to the stomach and the animal will regurgitate food usually shortly after eating. The food may also be inhaled into the airways causing pneumonia and cough. 

Please Read More About Vomiting From the WSU Vet School:

NB-Gastrointestinal problems like vomiting and diarrhea are two of the most common problems seen in veterinary practice.  Species-appropriate diet and homeopathy can help tremendously.  Vitamin and other nutrient supplementation are good support while the body reestablishes a strong equilibrium.  Once dynamic balance is restored, symptoms resolve.--Dr. Jeff

Please note: The information provided here is intended to supplement the recommendations of your veterinarian. Do not disregard veterinary advice or delay treatment based on information on this site. Nothing can replace a complete history and physical examination performed by your veterinarian. -Dr. Jeff