For some, all it takes is a really hot day; for others, excessive exertion combined with a really hot day. Heat stroke occurs when the body is unable to cool itself efficiently, sufficiently.
Caught early, proper first aid can keep a "heat illness" from developing into heat stroke.
If left untreated, heat stroke can be deadly. Those most vulnerable are the very young and old, but no one is immune.
Some of the initial symptoms include: High body temperature, lack of sweating, nausea, rapid breathing, elevated heart rate.
For your mandatory summer reading, here's the Mayo Clinic's guide to recognizing the symptoms of heat illness and heat stroke, and measures that should be taken immediately.
And remember, other animals can suffer heat stroke as well. Dogs particularly, since they join us in so many outdoor pursuits. Here's one online resource to help you help your buddy.