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 dermatologist is a doctor who has extensive training and knowledge of the skin, scalp, hair and nails and treats conditions that affect those areas. These doctors will evaluate any abnormality, blemish or lesion on the skin in order to determine the cause and will determine a course of treatment.
Dermatologists provide patients with full body scans in order to identify any signs that are indicative of an illness that requires treatment, such as skin cancer. These specialists may also provide cosmetic services, such as mole removal, scar diminishing treatments and even botox and face lifts.

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.