A stroke occurs when the blood supply to part of the brain is suddenly blocked (ischemic stroke) or when a blood vessel in the brain bursts, spilling blood into the spaces surrounding brain cells (hemorrhagic stroke). Brain cells die when they no longer receive oxygen and nutrients from the blood or when there is sudden bleeding into or around the brain. Ischemic stroke is the most frequent cause of stroke and is responsible for about 80 percent of strokes. These blockages stem from three conditions: the formation of a clot within a blood vessel of the brain or neck, called thrombosis; the movement of a clot from another part of the body such as the heart to the neck or brain, called embolism; or a severe narrowing of an artery in or leading to the brain, called stenosis.
(Source: Adapted from http://www.ninds.nih.gov/disorders/stroke/preventing_stroke.htm#Risk%20Factors)
A TIA (transient ischemic accident) is a "warning stroke" or "mini-stroke" that produces temporary stroke-like symptoms but no lasting damage. The symptoms occur rapidly and last only a short period of time, usually less than 5 minutes. TIAs are significant because they signal that a stroke is likely to occur. More than one-third of people who have a TIA will suffer a stroke in days, weeks, or even months later. A TIA should never be ignored. If symptoms appear, CALL 9-1-1 TO GET MEDICAL HELP IMMEDIATELY. A doctor should determine if a TIA or stroke has occurred, or if it's another medical problem with similar symptoms. Prompt medical or surgical attention to these symptoms could prevent a fatal or disabling stroke from occurring.
(Source: Adapted from http://www.strokeassociation.org/STROKEORG/AboutStroke/TypesofStroke/TIA/TIA-Transient-Ischemic-Attack_UCM_310942_Article.jsp)