Some of these are available in composites, which are sometimes partially made of wood sawdust or chips, and partially plastic. As far as we know, they last longer than wood, and require much less paint or preservatives, if any. I think they are a good idea, but they are, as you might expect, more expensive. Eventually, composites will more often be made of recycled materials, because it is possible and oil for plastic will get more expensive. Did you know that Maine is the home of some of the nations leading experts in composites, and many are manufactured here?
Regular solid wood clapboards are usually made of cedar. They have two sides: one is smooth and the other is less smooth. If you are planning on staining them, you might get a more interesting look if you expose the rougher side. If you are going to paint them, the smooth side is usually favored.
It is important to buy stainless or rust proof nails when fastening clapboards or shingles, or expect that the rust will penetrate your stain or paint eventually and leave rust lines on your siding.