The PCD (Pitch Circle Diameter) is the diameter of a circle (usually stated in millimetres) drawn through the centres of all the bolt holes.
The smart Fortwo and Roadster have the same PCD, 112.5mm.
The smart Forfour PCD is 114.3mm (4.5 inches).
The offset is the distance (in millimetres) from the centre of a wheel cross section.
As it moves away from the car the number increases from zero.
If it moves in towards the car it will go into minus numbers.
This picture is an example of one of the offsets found on the smart, for a full list click here.