Tailgating is the practice of driving on a road too close to a frontward vehicle, at a distance which does not guarantee that stopping to avoid collision is possible. A minimum 2 second following rule (under optimal conditions) is recommended by driver handbooks, therefore tailgating can be defined as following closer than this distance. 
There can be several reasons for tailgating:
- Tailgating can occur because of a lack of perceived risk in so doing. Thus, it is done unconsciously or negligently, very often by people who consider themselves safe drivers and generally obey the other rules of the road.
- In its worst form, it can be a particularly violent form of road rage and a form of intimidation. An example would be where the tailgating driver (the driver in the following vehicle) threatens damage to the leading vehicle and its occupants by driving aggressively — perhaps also with use of headlights and horn — to bully the leading vehicle's driver to get out of the way. The driver being tailgated might not wish to comply, especially if doing so would involve breaking the law, such as by increasing speed beyond the speed limit or changing lanes without due regard for safety. Note, however, that in some jurisdictions flashing high beams is a normal and polite method used to signal the intention to overtake. Tailgating can also be dangerous to the tailgater, especially if he or she is driving closely behind a large vehicle (such as a tractor-trailer, or gas tanker). If the leading vehicle decelerates suddenly (such as when encountering a traffic jam, traffic lights, avoiding pedestrians, etc.), the tailgater has a high risk of causing a rear-end collision.
- A form of deliberate tailgating known as slipstreaming, "draft-assisted forced stop", or "draft-assisted forced auto stop" (D-FAS) is a technique which has been used by people known as hypermilers to achieve greater fuel economy. D-FAS involves turning off the engine and gliding in neutral while tailgating a larger vehicle, in order to take advantage of the reduced wind resistance in its immediate wake. Note that this practice is extremely dangerous: while tailgating itself is inherently risky, the danger of collision is increased with D-FAS as power for power brakes can be lost after a few applications of the brake pedal and, with older cars, the pressure that causes power steering to function can be lost as well.
Fighting against tailgating
Tailgating causes most rear-end crashes in South Australia. The United Kingdom and Australia are trialing certain road markings which can help resolve this problem. Consisting of an arrangement of chevrons, these remind the driver not to tailgate. 
- Tailgating: How Close Is Too Close (RearEndCollision.net)
- Tailgating Information (Government of South Australia)
- TRAFFIC AND CRIMES AMENDMENT (MENACING AND PREDATORY DRIVING) BILL (New South Wales parliament Hansard transcript where tailgating is noted as being an offense under the new legislation).
- Tailgating Information (Wisconsin Department of Transportation)
- "Two Dots to Safety" campaign[dead link] (operating in the United Kingdom and Europe, but also in Pennsylvania, Minnesota, Maryland, and Washington State)
- Police Force safety advice (Singapore)
- Report on an anti-tailgating operation (by the Hong Kong police)