From what I understand, Intro/Raceline are 2 piece welded...they have different size hoops and then they cut the face design out of a few inch thick piece of aluminum and weld that into the hoop.
Forged, I believe is all one piece, I believe they make the entire wheel from one solid block of aluminum...I'm not sure if they cut it or press it into shape with some kind of high pressure machine.
American Forces are considered forged. They only make wheels for lifted truck because I guess the way their machine/or model is setup is only for that specific offset/width. That's why they don't make wheels for lowered trucks. Intro can make anything. Just tell the the width and backspacing and you can have a 20x12 for a slammed truck or a lifted truck.