A veterinary specialist is a veterinarian who has completed additional training in a specific area of veterinary medicine and has passed an examination that evaluates their knowledge and skills in that specialty area. Currently, there are 21 AVMA-recognized veterinary specialty organizations comprising 40 distinct specialties. Veterinarians can be specialists in behavior, ophthalmology (eye diseases), internal medicine, surgery, dentistry and many more areas. The specialty organizations are referred to as "colleges," but they're not schools or universities.

The specialist's expertise complements that of your veterinarian. You may be referred to a veterinary specialist if diagnosing or treating your pet's health problem requires specialized equipment and/or expertise that your veterinarian does not have. 

It's critical that you, your veterinarian and the veterinary specialist communicate and work together to provide the best care for your pet.

(https://www.avma.org/public/yourvet/pages/default.aspx)