A stroke is a medical condition in which the blood flow to our brain stops, causing the death of blood cells. There are a number of causes of stroke, including specific medical conditions (cardiovascular diseases, diabetes, high cholesterol, high blood pressure, etc.), smoking (first hand or second hand), obesity, heavy drinking, recreational drugs, etc. An example of a medical condition causing stroke is atherosclerosis, in which plaque (fatty substances) builds up on the inner side of our arteries, thereby choking the blood supply to our organs. This can cause a transient stroke, also known as ischemic stroke. High blood cholesterol levels increase the risk of plaque buildup. There can also be conditions in which a blood vessel ruptures inside the brain, causing to hemorrhagic stroke. This is generally caused by very high blood pressure or overuse of blood thinning medicines (such as heparin and warfarin).
A healthy lifestyle, proper diet and regular exercises are some of the ways we can reduce the risk of strokes.
Hope this helps.