What is 'aquaplaning' (aquaplanage / hydroplanage) and how should a driver react if it occurs?

1. A layer of water builds up between the tyres and road, causing complete loss of steering and braking; the driver should ease off the accelerator, avoid abrupt steering or hard braking, and keep the wheel straight until tyres regain grip
2. The vehicle's radiator overheats; the driver should immediately accelerate to blow air through the engine
3. The vehicle's brakes become locked; the driver must pull the handbrake with maximum force
4. The windscreen wipers stop working in heavy rain