Fellowships. Some specialties require fellowship training (beyond residency) - so there may not be interns for that specific fellowship.
Internship. Internship refers to the first year of postgraduate medical training. It can be part of a residency requiring additional years' training, or "rotating" if the doctor plans to transfer to a definitive specialty program beginning the second year. Fellowships are more advanced, and are in subspecialties. By this time the doctor is past internship & and several years' basic residency training.