What is aquaplaning (hydroplaning), and what should you do if it occurs while driving in heavy rain?

1. Tyres ride up on a sheet of water losing road contact; ease off the accelerator and hold the steering steady without braking hard
2. Water enters the engine air intake; accelerate hard to blow the water out
3. Brake drums become wet; pump the brakes continuously while speeding up
4. Steering mechanism locks mechanically; turn the steering wheel forcefully back and forth