Trucks

Attached to the deck are two metal (usually aluminum alloy) trucks, which connect to the wheels and deck. The trucks are further composed of two parts. The top part of the truck is screwed to the deck and is called the baseplate, and beneath it is the hanger. The axle runs through the hanger. Between the baseplate and the hanger are bushings, also rubbers or grommets, that provide the cushion mechanism for turning the skateboard. The bushings cushion the truck when it turns. The stiffer the bushings, the more resistant the skateboard is to turning. The softer the bushings, the easier it is to turn. A bolt called a kingpin holds these parts together and fits inside the bushings. Thus by tightening or loosening the kingpin nut, the trucks can be adjusted loosely for better turning and tighter for more control.

Longboard specific trucks are a more recent development. A longboard truck has the king pin laid at a more acute angle (usually between 38 and 42 degrees) to the deck, this gives a lesser degree of turning for the same tilt of the deck. This adds stability and allows riders to go much faster while still maintaining stability and control.

Source: wikipedia