Talking about and buying hardwood lumber comes with its own language.  but it can be split down into 2 categories.      By the quarter ( Four Quarter, 4/4 ) and by the board foot.  So what is a quarter, what is a board foot?

Four Quarter, 4/4, 8/4 ....

by the quarter (written as x/4) is the way thickness of lumber is explained.  where each quarter is a quarter inch in thickness.  so 4/4, or "four quarter" lumber is 1 inch thick. 8/4 or "eight quarter" is 2 inches thick.  and so on.

Board Foot

A board foot (/bf) is a piece of lumber that measures: 12 inches wide, 12 inches long and 4/4 ( 1 inch ) thick.  So a board that is 4 inches wide, 3 feet long and 1 inch thick, would be 1 board foot