For some reason the post form did not accept the graphic link. I'm adding it here for reference. This is a picture I took of the book graphic using my cell phone and posted to a dropbox folder.

https://www.dropbox.com/s/1aiqtd2b5tkbn9x/abos-gaff.jpg

Gaff rigs and lug rigs both use quadrilateral (4-sided) sails, but gaff rigs use a gaff to support the head of the sail, and lug rigs use a yard. You can no doubt find more precise definitions for gaffs and yards by googling a bit, but gaffs are attached to the mast via jaws on their forward ends, as in your JPG file, and yards cross the mast, with most of their span behind the mast, but some also in front of it. Gaffs normally are raised with two halyards, a peak halyard and a throat halyard. Yards are normally hoisted with a single halyard, which is attached to the yard slightly forward of its midpoint. Is that of any help?

Old Yeller 

@Old Yeller, thanks for the explanation. I had not noticed the difference. That helps.