1.Bow, describes the lengthwise curvature for board-end to end along its face.
2.Twist, means that all of a boards corners won't lie equally flat.
3.Crook, all the curvature runs end to end alone its edge.
4.Cup, is when a board no longer flat from edge to edge. Cup always occurs in the opposite direction of a flatsawn board's annual growth rings.
Although not a distortion like any form of warp,
5.Checking refers to the Small splits along the grain. You'll most often see checks in the ends of boards but they can occur on surfaces too. This is due to the fact that wood dries 10 times faster along it length compared to drying across the width.