A cerebrovascular accident, or stroke, occurs when blood flow stops reaching brain tissue. There are several different causes for the cessation in blood flow. The different causes determine how much brain is involved in the stroke. If the entire brain is involved it is referred to as a "global" stroke; if a specific region of brain is involved it is referred to as a "focal" or "territorial" stroke.
Global strokes occur when the entire brain receives no blood flow. The most common scenario for this is after a major trauma (ie: car accident) in which a person looses a significant amount of blood. The extremely low blood pressures that result from the loss of blood lead to decreased blood flow to the entire brain. The result is decreased oxygen and nutrient delivery leading to neuron death.
In contrast, the two other main causes of stroke affect specific "territories" of the brain. The territories affected depend on the specific blood vessel involved. The first cause of territorial stroke is referred to as a thrombus. A thrombus occurs when a clot (the technical term for clot is, in fact, a thrombus, but for the sake of explanation we'll use the term "clot") forms in a blood vessel that supplies brain tissue. This is similar to what happens in cardiac infarction (ie: heart attacks). Thrombi are most commonly caused by atherosclerotic disease of the cerebral blood vessels.
The second main type of territorial stroke is called an embolus. An embolus is similar to a thrombus because both involve blood clots. However, an embolus is a fragment of a blood clot (thrombus) that originally formed in another part of the body. Those fragments break free from the original clot and travel to brain blood vessels where they get lodged. If treatment is not sought quickly, the result is a stroke.