The stroke occurs when an artery bringing oxygen to the brain is blocked off. This can occur because a blood clot can travel from elsewhere in the body until it lodges in a blood vessel which is smaller than the clot. Plaque, a combination of calcium, cholesterol and other substances, can also build up in the larger arteries in the neck. A piece can break off and travel further into the brain where it can lodge in a smaller vessel, blocking it off.
There are other causes of stroke, but the end result is the same: loss of function due to the brain being deprived of oxygen. In general, the larger the blood clot, the larger the amount of brain that is damaged, and the more significant the symptoms the patient experiences.
In this video, Dr. Ahuja covers the common causes of a stroke: