A tuk tuk is a small three wheeled taxi in common use throughout Thailand (and apparently in India as well). The name "tuk tuk" comes from the sound of the the LPG fueled engine that powers the vehicle, and sounds similar to a demented chainsaw that has been involved crossbreeding experiment with a wasp's nest.
Tuks tuks are essentially three wheeled motorized samlors used in and around major urban centres. The machines are based around a tricycle design, and steered via a motorbike handlebar system. The passenger section of the vehicle is usually a padded foam bench surrounded by a highly ornate steel "fence".
Usually painted a garish shade of blue or orange, tuk tuks are a cheap and highly efficient way of moving around busy cities whilst receiving a healthy dose of carbon monoxide poisoning. In peak hours the tiny machines can often weave in and out of traffic faster than any car, but the drivers are expert at picking out a farangs, and they will try to take you for all you're worth, but once you've learnt how to haggle you might only pay 2-3 times the local rate.
Due to the lack of inbuilt safety gear and the fact that tuk tuk drivers have a bad habit of driving the wrong way up one way streets during rush hour, there are quite a few tuk tuk accidents, and most of those result in serious injuries to the passengers. Because of this the sale of new tuk tuks is allegedly illegal in Thailand, but this doesn't seem to reduce their numbers.