Wheel sizes are expressed as WW x DD sizes. For example 6 x 14. A 6 x 14 wheel is has a rim width of 6 inches, and a rim diameter of 14 inches. The width is usually below the width of the tire for a good match.
The 'Offset' of a wheel measures the distance between the wheel center line and the wheel mounting face and is measured in millimeters (Diagram 1). It is extremely important that wheels of the correct offset are used in order to maintain the correct track of the vehicles. Offset can be Positive or Negative (Diagram 2). Most Modern vehicles are front-wheel drive, and as such generally require positive offset wheels. The main exception to this rule is 4x4 vehicles, which often use negative offset wheels.
PCD - pitch circle diameter
The P.C.D. can be defined as the diameter (in millimeters) of an imaginary circle drawn through the center of the stud holes on the wheel and/or the vehicle wheel hub. When new wheels are required, it is essential that they have the correct P.C.D. for the vehicle concerned.