Mesothelioma is an uncommon form of cancer, usually associated with prolonged exposure to asbestos, which affects the pleura, a sac which surrounds the lungs, the peritoneum, the lining of the abdominal cavity, or the pericardium, a sac which surrounds the heart.

Mesothelioma is a rare form of cancer in which malignant (cancerous) cells are found in the mesothelium, a protective sac that covers most of the body’s internal organs. Most people who develop mesothelioma have worked on jobs where they inhaled asbestos particles.

The above was extracted from Wikipedia at http://www.wikipedia.org. The rest of the article is excellent on the details of the disease.