Vitiligo is a depigmenting disorder caused by the loss of melanocytes, the cells that make pigment. It is not uncommon occurring in about 1% of the population. More people have it than you might expect since it is not as apparent in fair people.
There are both familial forms and nonfamilial forms.
Vitiligo is an auto-immune disease which means that the body is producing antibodies which are attacking the melanocytes. Thus, it is associated with a number of other auto-immune diseases including thyroid disease (Hashimoto's thyroiditis), pernicious anemia, Addison's disease, and Diabetes Mellitus.
The appearance of vitiligo is a stark white area with a well demarcated border. There aer no other skin changes such as scaling. If you have been in the sun, however, the area can be red rather than white since you lack the normal pigment cells to protect you from a sunburn. Sometimes, the border can be hyperpigmented. Early cases are not as apparent and there may be just mild hypopigmentation. Areas most likely affected are the areas around the eyes, mouth, genital, as well as the elbows, knees, armpits, inguinal folds, and forearms. Frequetnl;y, the lesions of vitiligo develop symmetrically on the trunk and extremities.
Hope this is a decent summary.