There are two types of tailgating, one good, one bad.
Bad tailgating occurs when driving and one car follows the other very closely. This form of tailgating leads to auto accidents because the following distance is covered in less than the trailing driver's reaction time. The fact that the trailing driver always gets the ticket is scant compensation for the accompanying whiplash.
Good tailgating takes place before football games and at road racing events. Essentially it is a party or pic nic set around the tailgate of an SUV, station wagon, or other motor vehicle capable of carrying coolers. Because beer is heavy, at least in quantities sufficient to last the weekend, it is better to keep the coolers in the vehicle. Lower the tailgate, set out the mustard and begin consumption. One American tradition that deserves to spread.
Tailgating can be as simple as opening the trunk lid, or can become quite sophisticated. Many tailgators base their parties out of specialized vehicles, often painted school colors. Motorhomes, converted school busses are often chosen. Grilling is encouraged, and pig roasts are not unknown. Many tailgators use portable televisions with satellite dishes to follow the sporting event. Menus can be as simple as the humble PB&J to rib roasts with chili on the side. Beer may be bottled or by the keg.
Because even a humble station wagon can carry a lot of goodies, a large, well-stocked party can be set around a single vehicle. Tailgating has made partying mobile, with the proviso that some sanitary provisions should be made available for beer recycling.