I know the question has already been answered, but there is some extra info here. I was experiencing trouble with wood lingo a few months ago. I found these simple explanations very useful.
Thought I'd share.
what is 4/4 and 8/4?
When a log is first felled and sliced into rough boards, the caliper on the dial used to determine the thickness of the board is divided into quarters of one inch. Therefore, the miller will set his thickness to 4/4 if he wants an inch thick board, or he will set it to 8/4 if he wants a two inch thick board, 12/4 if he wants 3 inches, and so forth.
What is a Board Foot?
You figure a board foot the way you would figure a square foot. For a 1" thick or 4/4 board a square foot is the same as a board foot. You multiply the length times the width and divide the result by 144.
For a 2" thick or 8/4 board, a board foot equals square feet times 2. You multiply the length times the width and divide the result by 144, then multiply the answer by 2.
For a 3" thick or 12/4 board, a board foot equals square feet times 3.
Board feet are always determined by the rough rather than the finished thickness.
What is S3S or S4S?
An S3S board is one that has been planed to 25/32 and straight lined on one edge.
An S4S board is one that has been planed to 25/32 and straight lined on both edges, and is square.
An S2S board is one that has been planed only and may have a curved edge on both sides.
All the above info wa gleemed from this site: The Lumber Lady