"Curly" woods have fibers that are wavy instead of straight, which causes them to reflect light in interesting ways. Many kinds of wood can exhibit this trait; it's quite common in maple, where the effect can be spectacular, and is sometimes seem in other hardwoods such as birch and cherry (I can't say I've ever seen it in pine, but the world's full of surprises). It's a genetic quirk, kind of like wavy hair in people, so some trees have it and others of the same species don't. Curly lumber usually sells for a premium price, and items made from it are more expensive both because of the cost of the wood and the difficulty of working with it.
Just FYI, there are various forms these patterns (collectively called "figure") can take, each with its own name:
Curly:
Birdseye:
Quilt:
Ribbon:
Pommele:
