PCD (Pitch Circle Diameter) is an imaginary circle through the wheel studs that tells you the distance between them, on a GC8 & GDA its 5x100 or 5 studs in a 100mm diameter circle, on the 05-07 STI its 5x114.3 or 5 studs in a 114.3mm dia circle
Offset describes where the the rim contacts the hub & how far it is from the centre line of the rim. this will effect things like its ability to clear bigger brakes (03 GD for example wrx is +48 for 4 pots or +53 for Brembos), ability to clear your struts, clear the bodywork etc
clear as mud ?