A pathologist is a physician who specializes in the causes and paths taken by different diseases in order to accurately diagnose an illness.
Pathologists diagnose and determine the characteristics of a disease through the study of biopsies of diseased tissue or of bodily fluids. For example, a pathologist will look at a biopsy of a skin lesion in order to diagnose or rule out skin cancer. A pathologist will also look at a Pap smear in order to check for a gynecological cancer like cancer of the uterus.
In addition to determining the cause and development of a disease, these specialists also study the changes a disease makes to a body and the consequences of those structural changes.

A hematology specialist is an expert in disorders of the blood, the blood forming organs and bone marrow. These doctors diagnose, treat and work to prevent diseases that affect the production of blood and its components, as well as the ability of the blood to perform its many functions, such as coagulation and carrying oxygen to the lungs and tissue.
Hematologists diagnose and treat blood disorders, such as anemia, hemophilia, leukemia, sickle cell anemia, deep vein thrombosis and pulmonary embolism.
Depending on the condition, hematology specialists may treat a patient with a blood transfusion, stem cell transplantation, bone marrow transplant, radiotherapy, anticoagulation therapy or medication.

Similar to what pathologists do, cytopathologists analyze specimens in a lab to help your doctor diagnose your condition. However, cytopathologists primarily analyze cells from tissues, tumors and lesions in order to detect disease, whereas pathologists primarily focus on bodily fluid samples.
The most common example of this is pap smears, which cytopathologists use to detect and study the precursors to cervical cancer in women. Additionally, they help oncologists detect blood cancers and blood disorders. You can think of all pathologists as a type of lab consultant for doctors. You consult with your doctor and then he or she often consults with a pathologist who uses lab testing to confirm what they suspect your condition might be.

A dermatopathologist is an expert in both dermatology and pathology. They specialize in diseases related to the skin, but where a dermatologist would examine a potentially-cancerous mole by looking at it, a dermatopathologist would examine your skin cells in a lab, more like a pathologist.
Beyond just skin cancer, dermatopathologists also study a wide range of skin conditions, including everything from warts, moles, spider veins and herpes, to dermatitis, rashes, vitiligo and other conditions related to skin discoloration.