Cushing's syndrome results from excessive cortisol production (hypercortisolism) from the adrenal cortex. This can be due to adrenal tumors, pituitary ACTH-secreting tumors, or exogenous corticosteroid administration. Symptoms include weight gain, moon face, buffalo hump, and hyperglycemia. Decreased cortisol would cause Addison's disease, not Cushing's.