A neurologist is a physician who diagnoses and treats disorders of the nervous system which is comprised of the brain, spinal cord and nerves. These doctors do not perform surgery, but refer patients to neurological surgeons when they determine that surgical intervention is necessary.
Some of the conditions that neurologists diagnose and treat are epilepsy, aneurysms, hydrocephalus, Parkinson's disease, multiple sclerosis, stroke, spinal disc herniation, and spinal disease.
In addition to using diagnostic tests like MRI, CT scans, EEG and EMG, neurologists also employ neurological testing to gauge muscle strength and movement, balance, reflexes, sensation, memory, speech, and other cognitive abilities.

An addiction medicine specialist is a doctor who treats patients with addictions to substances like drugs and alcohol or, in some cases, behaviors like gambling. Many of these specialists also treat the diseases that stem from these addictions.
These physicians are specially trained to prevent and treat the disease of addiction via psychiatric means, or other fields of medicine like family or internal medicine.

A sleep medicine specialist is specially trained in diagnosing and treating disorders involving sleep.
Sleep disorders such as sleep apnea, restless leg syndrome and insomnia are very common and can often cause other serious health issues, such as depression, asthma, and migraines. Sleep medicine specialists often work in sleep centers where they observe a patient while sleeping and monitor brain waves, behavior, and vital signs in order to identify the causes of sleep disturbance, or an inability to sleep (insomnia).
Sleep medicine specialists treat patients through advising on sleep hygiene, providing cognitive behavioral therapy, using light therapy, or medical sleep aides.

A psychiatrist is a doctor with specific training in the diagnosis and treatment of mental illness.
He or she can not only provide the counseling necessary to both diagnose and treat a patient, but can also prescribe medication when needed. In some cases, a psychiatrist will only provide the medication and the counseling will be provided by another healthcare specialist, like a certified counselor or psychologist.
Like other doctors, psychiatrists employ diagnostic tools like CT scans and MRI in order to observe the structure and function of a patient's brain.
Once a diagnosis is made, these specialists may use behavior or cognitive therapy in order to address the patient's condition, or a multitude of other types of therapy, in conjunction with or in place of medication.

A family practitioner is a doctor who specializes in caring for people of all ages, at all stages of life. Rather than focusing on the treatment of one disease or patient population, family practitioners are often the doctors that people see for their everyday ailments, like cold and flu or respiratory infections, and health screenings. When necessary, family practitioners will provide referrals for conditions that require the expertise of another specialist.
The doctors may also provide physicals, inoculations, prenatal care, treat chronic diseases, like diabetes and asthma, and provide advice on disease prevention.