What is 'hydroplaning' (or aquaplaning) and how should you respond if it happens?

1. Tires lose contact with the road on water; ease off the accelerator and steer straight without braking hard
2. Tires overheating on dry asphalt; brake hard to stop and cool them down
3. The car engine overheating due to water in the radiator; turn off the ignition
4. A sudden loss of brake pressure in wet weather; pump the brakes continuously