Bushings are the small pieces of urethane (urethane is a substance similar to rubber) that fit around the kingpin of a truck and rest on either side of its hanger. The bushing that fits in between the baseplate and hanger of a truck is the “boardside” bushing because it rests closer to your skateboard. The other bushing is called your “roadside” bushing and it rests on top of your hanger and nearer to the road.
Bushings come in an array of hardnesses to affect the turning characteristics of your trucks. The hardness of a bushing is measured in what we call the “durometer.” A durometer is indicated by a number followed an “a,” for example, one might have bushings with an 85a hardness. A higher number indicates a harder bushing while a lower number indicates a softer bushing.
Riptide offers bushings starting at 60a, which is extremely soft, ranging all the way to a 97.5a hardness, which would be relatively hard. It is important to note that when dealing with Riptide bushings that they generally feel a little bit softer than they test, as indicated on their website, and I definitely agree. Which means if you generally ride an 85a bushing you should consider buying a harder riptide bushing, such as an 88a or 90a.
In addition to coming in different hardness's bushings come in a couple different shapes. I would say that there are two shapes which are widely accepted as the most common; the barrel and the cone. In fact many trucks come pre equipped with some sort of a barrel/cone combination.