Traditionaly only a few types of wood are used for guitarbuilding. The typical guitar has a spruce top, sides and back made of rosewood, mahogany or maple, a neck made of mahogany or maple and a fingerboard made of ebony or rosewood. Yet this choice isn't compelling dictated, other wood types give excellent guitars too. Additionaly increasing shortage of certain tropical wood types force to look after alternatives. You can use cherry, walnut, oak, ash, beech, alder and many more types for back and sides and even for the top.
Wood for guitarbuilding should be almost free of defects and beautiful too. Straight grained wood is prefered for the top but whats more important is the stiffness. If a wildly grained wood is stiffer than a straight-grained one its the better choice. The same is true if a flatsawn top (horizontal annual rings) is the stiffer one.
A piece of wood, like that one in the drawing above, is tared apart on a band saw (so called resawing) and the two halfs are opened like a book and glued together to achive a symmetric top, back or sides. One reason for this is that suitable logs, thick enough for a one piece top or back, are very,very seldom.
Another quality factor is the lack of runout. If the piece of wood is sawn perfectly parallel to the middle axis of the log no grain is cut, the piece of wood is stronger. But this is almost impossible. By splitting the wood a good compromise is achieved.
Here's an interesting idea from Michael Bozalina: "Look for old furniture that the wood can be used for guitar building. I was recently given some wood from dismantled library shelves (they were beng replaced with sleek metal shelves. The shelves were made in the late 40's out of East coast rock maple (USA). The maple is beautiful, some is flamed, some is birds eye, some just plain but all beautiful. Think about it wood that is decades old. Instant seasoned guitar."