Hardwood is a type of lumber that comes from deciduous trees. Oak and walnut are the most prevalent hardwoods. Other hardwoods include:

Size
Hardwoods come in quarter-inch increments. Rough (unfinished) hardwood is actual size. Surfaced hardwood of an inch and a half or less in size is actually 3/16" smaller; larger sizes are 1/4" inch smaller than advertised. The face width is usually also actual side; hardwood is normally only surfaced on two or three (FAS) sides.

Grading
Hardwood grading is simpler than softwood grading. Grades are based on the appearance of the worst side, bearing in mind that the board is assumed to be cut into pieces 2 to 7 feet long.

First and Second (FAS)
The highest grade. It must be at least 83.3% (5/6) clear on the poorest face, and be at least 6" wide. FAS board is normally required for natural or stained finish work.
Select - No. 1 Common
Wood at least 3" wide, and 4 to 16 feet long, whose worst face is at least 66.66% (2/3) clear.
Select - No. 2 Common
Select - No. 3 Common

For more information on hardwood grades, you can acquire:
Hardwood Rule Book (USD 6.00)
National Hardwood Association
P. O. Box 34518
Memphis, TN 38184