It's easy to fix a damaged hardwood floor, but you'll need to know what sort of finish it has. To find out, wipe the floor in an inconspicuous spot with an acetone-based, nail-polish remover. Wax, shellac and varnish finishes will come up, but polyurethane won't.
To remove scuff marks on a waxed floor, spray the area with vinegar-based cleaner, and allow it to soak. Then wipe lightly with 0000-grade extra-fine steel wool (image 1).
You can also use paste wax to clean waxed floors (image 2). On polyurethane-coated floors you can remove scuff marks with a pencil eraser.
To conceal scratches, use a touchup stick (image 1), which is similar to a wax crayon. If the touchup stick doesn't work, try a felt-tip touchup pen (image 2), which contains wood stain rather than ink. It won't matter much if the colors are slightly off, because the wood probably has different colors anyway.