In a short explanation, every wheel adapter is also a wheel spacer, but a wheel spacer is not a wheel adapter. Wheel spacers are used for clearance problems with brakes, tie rods, and other suspension components. Wheel adapters do all the same things as wheel spacers do. However, they can change the bolt pattern and center bore diameter to allow you to bolt up wheels that are not made for your car. Both wheel spacers and wheel adapters push your existing wheels out more to fill the wheel-wells or your custom wide-body fenders properly. They also give your car better cornering, aerodynamics, and look.