The caster effect refers to the tendency of wheels, such as those on a shopping cart or motorcycle, to align with the direction of travel due to an offset between the point of contact and the pivot axis. This alignment occurs because a sideways wheel creates a torque force that straightens it out. The distance from the contact point to where the axis intersects the pavement is known as "trail." There is some confusion regarding the spelling, with "caster" being the correct term for the effect, while "castor" refers to oil. Clarifying the terminology can aid in finding more accurate information online.