Bow
Bow, or bowing, refers to bending of the board along the grain lines. Instead of laying flat, a board that is bowing will have both ends in the air, though flatness across the face will not be impacted. Bowing comes about in the drying process of wood and can be avoided through proper drying time and practices.
Cup
A cup, or cupping, is similar to a bow but differs in that it warps along the sides of the board (edge to edge, turning the board into a “U” shape) instead of lifting at the ends of the board like with bowing. Cupped boards can be ripped into smaller boards and planed though forcing the full board to become flat will cause cracks.
Cracks or checks
Cracks and checks are another wood defect that occur in the drying process. As wood shrinks when it dries, the checks and cracks arise. As visible breakage along the length of a timber piece, they are often a cause for concern though in reality, checks and cracks are a natural “side effect” of wood drying, are likely to close or shrink overtime and will not compromise structural integrity.
Twist
Twist, or twisting, is more of a general term for a board that bends in various directions. Twisting may occur because of the grain pattern but it can also occur from improper drying techniques.