Freehub Bodies: A freehub is an axle assembly, used notably for the rear axle of bicycles. Freehubs incorporate a ratcheting mechanism. Separately, a set of sprockets (called a "cassette") are mounted onto a splined shaft on the freehub. In contrast, a freewheel contains both the chain gear cogs and a ratcheting mechanism in a single unit. In many high-end and midrange bicycles, freehubs have replaced the traditional, heavier and more exposed freewheel systems. Source: Wikipedia.