An internist takes care of lots of different problems and is a general doctor for the heart, kidneys, nerves, stomach, hormones, etc. An endocrinologist is an internist who received special training (minimum of 2 years) in the field of endocrinology. A diabetologist is an endocrinologist super specializing in diabetes, which is one of several conditions within the field of endocrinology.
Your interenist can recognize the symptoms of Diabetes and get you on Metformin (the diabetes "starter" drug of choice). An Endocrinologist or a diabetologist is a MUCH better choice to take you beyond the metformin when this drug and your dietary controls stop working or simply aren't doing enough.