This occurs because the blood vessel has gotten clogged or because the blood vessel has ruptured. Since the blood vessels bring oxygen to the tissues, the brain cells that are deprived of this cannot function properly. If the brain cells can't function, the part of the body they control doesn't function. This presents as various symptoms of strokes.
Brain cells can die from lack of bloodflow, and thus effects of stroke can be permanent.
TIA stands for Transient Ischemic Attack. It occurs when blood flow to the brain is blocked temporarily, and bloodflow resumes spontaneously after a realtively short peroid of time. If you have had a TIA your risk for stroke is 10 times higher for having a stroke. Thirty six percent of those who have a TIA go on to have a stroke. Fifty percent will have the stroke in the first year following the TIA, and out of these 20 percent will occur in the first month.