Stroke. Org. For good info.
Think F.A.S.T.! The classic symptoms of an ischemic stroke are facial droop, arm drift (unequal strength in the upper or lower extremities), and slurred speech. The "t" is for the time the symptoms began. If you observe someone with these symptoms call 9-1-1 and get them to an emergency room immediately!
Varied. It depends on what part of the brain is involved. The general answer is sudden (over seconds) loss of neurologic function lasting >24 hours. A common scenario is weakness or paralysis on one side but symptoms can be subtle and require expert evaluation to sort, e.g. Change in behavior, double vision, sagging eyelid, vertigo, trouble speaking or understanding can be due to stroke or other causes.