The PCD, pitch circle diameter, refers to the distance (diameter) between the centre of the rim hole to the individual lugs. For example, PCD4x100 refers to 4 lugs, with a diameter of 100 milimetres between the centre to the individual 4 lugs.
The lugs simply indicate the number of holes on the rim, 4 lugs would refer to 4 holes, whereas 5 lugs would indicate that the rim or hub has 5 holes.