The list of sub-categories in medicine is pretty much the same as the list of specialists in medicine. This list is fairly complete, although there are one or two specialties that aren't listed such as sports medicine, and some fields such as radiology have more than one type of specialist. But for your purposes, this should be accurate enough.
Allergist or Immunologist - conducts the diagnosis and treatment of
Anesthesiologist - treats chronic pain syndromes; administers
anesthesia and monitors the patient during surgery.
Cardiologist - treats heart disease
Dermatologist -treats skin diseases, including some skin cancers
Gastroenterologist - treats stomach disorders
Hematologist/Oncologist - treats diseases of the blood and
blood-forming tissues (oncology including cancer and other tumors)
Internal Medicine Physician - treats diseases and disorders of
internal structures of the body.
Nephrologist - treats kidney diseases.
Neurologist - treats diseases and disorders of the nervous system.
Neurosurgeon - conducts surgery of the nervous system.
Obstetrician - treats women during pregnancy and childbirth
Gynecologist - treats diseases of the female reproductive system and
Nurse-Midwifery - manages a woman's health care, especially during
pregnancy, delivery, and the postpartum period.
Occupational Medicine Physician - diagnoses and treats work-related
disease or injury.
Ophthalmologist - treats eye defects, injuries, and diseases.
Oral and Maxillofacial Surgeon - surgically treats diseases, injuries,
and defects of the hard and soft tissues of the face, mouth, and jaws.
Orthopaedic Surgeon - preserves and restores the function of the
Otolaryngologist (Head and Neck Surgeon) - treats diseases of the ear,
nose, and throat,and some diseases of the head and neck, including
facial plastic surgery.
Pathologist - diagnoses and treats the study of the changes in body
tissues and organs which cause or are caused by disease
Pediatrician - treats infants, toddlers, children and teenagers.
Plastic Surgeon - restores, reconstructs, corrects or improves in the
shape and appearance of damaged body structures, especially the face.
Podiatrist - provides medical and surgical treatment of the foot.
Psychiatrist - treats patients with mental and emotional disorders.
Pulmonary Medicine Physician - diagnoses and treats lung disorders.
Radiation Onconlogist - diagnoses and treats disorders with the use of
diagnostic imaging, including X-rays, sound waves, radioactive
substances, and magnetic fields.
Diagnostic Radiologist - diagnoses and medically treats diseases and
disorders of internal structures of the body.
Rheumatologist - treats rheumatic diseases, or conditions
characterized by inflammation, soreness and stiffness of muscles, and
pain in joints and associated structures
Urologist - diagnoses and treats the male and female urinary tract and
the male reproductive system