Pheochromocytoma is a tumor of the (usually) adrenal gland. This tumor causes the secretion of massive amounts of adrenaline. The adrenaline causes severe hypertension, and may also cause some of the symptoms seen in severe fright (increased heart rate, pallor, sweating, headache, nausea etc.). The main treatment of pheochromocytoma is surgical removal. This must be done carefully, by first tying off the veins leading out of the tumor, then the arteries leading into it. This is done to prevent a massive adrenaline surge which could occur as the tumor is manipulated. Until the tumor is removed, patients are treated with alpha-adrenergic blockers and beta-adrenergic blockers to counteract the massive adrenaline doses being dumped into their system.
Pheochromocytoma is not usually cancerous, but is statistically associated with higher chances of having a tumor in another endocrine gland, such as the thyroid. It can be diagnosed through breakdown products in the urine.
For more information, check out http://www.mayoclinic.com/invoke.cfm?id=HQ01210 - thanks for the help, Chark!