According to Wikipedia:
A stroke is a medical condition in which poor blood flow to the brain results in cell death.
There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both result in parts of the brain not functioning properly.
Signs and symptoms of a stroke may include an inability to move or feel on one side of the body, problems understanding or speaking, dizziness, or loss of vision to one side.

Minor and Major Strokes

So, a stroke is a temporary interruption to the blood supply in the brain, that causes permanent damage, and the loss of some aspect of cognitive ability. But how much damage?
If very little damage is caused, with only minor effect, then it’s termed a minor stroke. This can’t be ignored, but taken as a warning to take action. If you don’t, you could get another minor stroke, or worse …
If major damage occurs, it’s termed a major stroke, and that often causes paralysis of part of the body, loss of speech, and other serious effects.

What can we do to reduce the chance of a stroke?

The National Health Service says that strokes can occur in two different ways:
• First, where there is narrowing or blockage of arteries.
The causes: smoking, high blood pressure, obesity, high cholesterol levels, diabetes, consuming excessive alcohol.
• Second, where high blood pressure bursts a blood vessel within the skull.
The causes: being overweight, consuming excessive alcohol, smoking, lack of exercise, stress.

The US Center for Disease Control and Prevention has these as the lifestyle factors that increase the risk of stroke: Unhealthy Diet, Lack of physical activity, Obesity, Too Much Alcohol, Smoking.

In summary, you can’t change some risk factors, like your age, but changes in lifestyle can vastly improve your odds of avoiding strokes, so in particular:
• If you smoke, stop!
• Keep your weight down
• Take regular exercise
• If you drink alcohol, do so in moderation
• Have a sensible diet, without excessive animal fat, sugar and salt