Skateboard trucks are made from several parts. The axle is the pin that runs through the trucks that the wheels will attach to. The hangar is the largest part - it is the metal, somewhat triangular area that the axle runs through. Hangars are often made of softer metal for better grinding, but not always. The kingpin is the large bolt that holds the hangar in place, and it runs through the middle of a rubbery donut called the bushing into the metal baseplate. Bushings come in various harnesses, and the kingpin can be adjusted to be either more or less stiff. Some skateboarders like loose trucks, meaning that they turn easier due to the kingpin being loser, and some skaters like their trucks tighter. It's a personal preference.
Skateboard trucks come in various sizes and colors, and many companies make trucks with special features. Like everything with skateboarding, it's up to the skateboarder when deciding what size or brand of trucks to buy. If you want some help, take a look at this Top 10 Skateboard Trucks list before you buy skateboard trucks.