Unfortunately, there are very few perfect pieces of lumber out there. If you are going to build a deck you might as well become familiar with the different types of defects so you know what to watch out for.
Defects in lumber are faults that detract from the appearance and utility of the wood. Some are present when the wood is produced and others develop or become more sever as the wood ages. As a general rule you can reduce the amount of defects by purchasing higher grade materials at the outset.
Properly drying the wood before it is installed will also help. Using protective stains on your deck can prevent some damage to the wood by slowing down the drying process in order to limit checking. Stains can also conceal visual defects by covering them with a consistent color throughout the deck.
Warping is any variation from a true plane surface on a piece of lumber. This occurs due to the differences between radial, tangential, and longitude shrinkage in a piece of wood or growth stresses that encourage the board to reform its original shape.