The offset is the measurement of the distance from the mounting hub surface to the middle (or centerline) of the wheel. Offsets are usually measured in millimeters. Positive Offset brings the wheel into the engine side of the vehicle and Less Positive, Zero or Truly Negative (-12, etc.) Offset moves the wheel out towards the fender side. Without the proper offsets, the wheel can have rubbing problems with either suspension or fender components, under load or when engaging a turn. Offset also can affect the steering geometry of a vehicle, leading to possible changes to torque steer or self-centering characteristics.
Backspace is a measurement of the distance from the mounting hub to the inner lip of the wheel. This measurement is usually in inches. Below is a backspace-to-offset conversion chart based on the width of the wheel.