Standard gas log sets are for fireplaces that have an opening on only 1 side. Most fireplaces are standard fireplaces and are only "open" are one end.
See-Thru gas log sets are for fireplaces that have openings on both sides (like a fireplace that has an opening facing your den and one facing your dining room). You can also place "See-Thru" gas log sets in peninsula fireplaces (openings on 3 sides). "See-Thru" log sets are "finished" on all sides so they look very nice from any angle. Standard gas log sets are only "finished" on the front side.