Lumber is
timber (
wood) which has been cut into various sizes for use in construction.
Lumber comes in three main varieties, plywood, softwood and hardwood. Each type of lumber is sized differently - a two by four is likely not actually 2 inches by 4 inches.
Most lumber is sold by the board foot (feet board measure) which is actually an arbitrary volume measure of 144 cubic inches, based on the named (not actual) sizing.
Lumber is also graded; these grades vary for hard- and soft- woods.