What exactly causes a stroke?

From Cognitive Neuroscience (pg. 134):

The most frequent cause of stroke is occlusion of the normal passage of blood by a foreign substance. Over years, arteriosclerosis, the buildup of fatty tissue, occurs in the heart. This tissue can break free, becoming an embolus that is carried off in the bloodstream. An embolus that enters the cranium may easily pass through the large carotid or vertebral arteries. But as the arteries and capillaries reach the end of their distribution, their size decreases. Eventually, the embolus becomes stuck, or infarcted, blocking the flow of blood and depriving all downstream tissue of oxygen and glucose. Within a short period of time, this tissue will become dysfunctional. If the blood flow is not rapidly restored, the cells will die.