Defensive Driving is a collection of techniques that a driver uses to notice situations which could be dangerous, early enough that action can and is taken to avoid them. Defensive driving is a combination of both noticing potentially dangerous situations, and modifying driving behaviour to allow for these situations.
The process of noticing potentially dangerous situations starts before the defensive driver gets into the car. The defensive driver checks the vehicle is safe, and that aids like turning indicators and brake lights are working, before starting the vehicle. Weather and road conditions are also checked, and mental allowances made that will be used during the trip.
Defensive driving involves treating a trip and the vehicle it will be made in, much the same as an airplane pilot treats his vehicle and journey. Checks are performed before the trip and during the trip, and a good visual lookout is kept all through the trip. In addition the defensive driver practises (at least mentally) reactions to possible unexpected situations
A defensive driver is always on guard for the possibility that someone else may make a mistake, and is aware of what is happening in an extended area around the vehicle so that as much warning as possible is received of potential hazards.
I recommend that everyone that drives a vehicle should attend a defensive driving course, and should periodically have a defensive driving instructor sit in the vehicle and advise on defensive driving habits. You would probably not want to take a trip in an airplane if the pilot had not been recently assessed on his pilot skills. Most of us spend less time driving our vehicles than a pilot spends flying his plane, so we get out of the habit of defensive driving even faster than a pilot does.
When you are driving keep the defensive driving basics in mind, and stay alert to notice any situations which could become dangerous. Work on the assumption that other people will make mistakes, and adjust your position, speed and direction so that you will be able to avoid any developing dangerous situation. Notice what is happening ahead of you in time to react safely, and around you so that you are aware of approaching and overtaking vehicles. Check around you before changing speed or direction.
Vehicle safety is not an accident. Learning defensive driving, practising the defensive driving techniques when driving, and having a defensive driving instructor check your driving periodically are the keys to avoiding preventable accidents.