Solid wood features distinctive growth rings that run all the way through the board and are visible on the cut end. Due to its higher cost (compared with other shelving materials), solid wood probably isn't the best choice if the shelves will be painted. It's a good idea to use solid wood only for shelves that will be finished with stain and placed in a room where looks and quality count.
A second option is plywood. The end of a plywood board will exhibit several bands of wood; it's created for strength and can actually support more weight than solid wood. (It's also sold in wider widths, like the 4' x 8' sheets often used for shelving projects.) The disadvantage to using plywood is appearance. It's not attractive and usually requires two to three coats of paint to cover its many imperfections. Plywood shelves are better used in a "low-visibility" area like the garage or basement. Plywood sheets finished with a thin veneer of oak, mahogany or maple can also be found. Like standard plywood, the end of the board will reveal multiple bands of wood, but the surface features an attractive grain pattern.
As with solid wood, however, the issue is expense: if you're planning to paint the shelves, it's better to use a more economical material. Particleboard, yet another shelving option, has become more appealing in recent years, since it's now manufactured to have a fairly smooth, even surface (free of large chunks). And, like plywood, it's available in large 4' x 8' sheets that are ideal for shelving. The drawback here? Particleboard won't have an attractive, finished appearance unless it's painted.
Laminate, however, is a low-cost option that doesn't require the hassle of painting. Laminate is actually finished particleboard that boasts a smooth white surface. It's sold in the shelving section of most home-improvement centers (along with solid wood, plywood, etc.), and it's fairly economical.