Mesothelioma is a form of cancer that develops in the mesothelium, and is almost always caused by asbestos exposure.

Pleural Mesothelioma – around the lungs (lung cancer refers to the lung)
Peritoneal Mesothelioma – around the abdomen
Pericardial Mesothelioma – around the heart

However, those are not the only places Mesothelioma can develop.

the pleura (thoracal cavity)
peritoneum (abdominal cavity)
pericardium (heart sac)
tunica vaginalis testis (male internal reproductive organs)
tunica serosa uteri (female internal reproductive organs)
visceral mesothelium (internal organs)
parietal mesothelium (body walls)

Cell Types of Mesothelioma simply refers to the mesothelium tissue that the cancer developed in.

Epithelial Mesothelioma – cancer of the epithelial tissues surrounding the heart, lungs and abdomen.
Sarcomatous Mesothelioma – cancer of connective tissues
Other cell types – Biphasic, lymphohistiocytoid and desmoplastic

You generally will not hear of the “cell type” unless you or a loved one has been diagnosed with malignant Mesothelioma.