There are a few types of heat crises that need first aid. The cause is the same, too much exposure to hot weather for too long, probably doing too much work with too little to drink. Heat emergencies exist on a continuum, from not very severe, to immediately life threatening.
First comes heat exhaustion, common from exposure to hot, humid environments in particular. signs include moist, pale, clammy skin, hefty sweating, usual body temperature, dizziness, annoyance and irritation, nausea and potentially vomiting, muscle cramps, and even fainting. The treatment is generally simple: move the person to a cooler environment, offer small, frequent amounts of cool fluids, remove excess clothing, and provide cool towels and rest. If the patient is not improving or continues to vomit, get further medical treatment.
Heatstroke is at the far end of the spectrum. Heatstroke is a potentially life-threatening emergency with the hallmark sign of markedly increased body temperature, potentially above 106 degrees Fahrenheit. The victim may have, warm, dry skin; tachycardia, and unconsciousness. This patient requires 911 and immediate advanced intervention. First Aid would involve calling 911, have the person lie down if possible, get clothes off, and start lowering the body temperature immediately by any means necessary. (The AHA video shows a couple of guys holding the heatstroke victim in a pool submerged to the chest. Bonus points for creativity.) Hosing the person down and applying ice packs would certainly be appropriate.