When carpeting a basement, you should also add a subfloor. Doing so will provide a cushion between the actual basement floor, usually concrete, and the carpeting. Subfloors prevent damage in the event of a water issue. They also give your carpet a more professional, uniform look and feel.
The best choice for sub-flooring in this instance, is a woven, rubber subflooring. The weave design will help any moisture to evaporate, or be cleaned with a Shop-Vac. The rubber will be rid of the possibility of mold and mildew growth. Aside from those benefits, the woven rubber subflooring will not break down as easily as wood or foam subfloor.