Most wood floors come from trees harvested for that purpose -- they're cut down with beautiful flooring in mind. And when a homeowner pulls up old, beat-up wood floors during a remodel, that old wood is typically considered trash and is discarded accordingly.
Two types of wood floors take a more eco-friendly approach. There's reclaimed wood, which uses re-finished, old wood floors and other building elements (old beams, for instance) to make new wood floors that have a rustic look. These are antique wood floors. Reclaimed wood can be used for other design elements, like staircases, trusses, counters or mantles.
Another green option is recovered wood. This type uses trees that have been cut down for other purposes, typically in clearing land for building purposes. In this case, recovered trees that were being killed anyway are used to create brand-new wood floors.
One thing to look out for in both of these cases is the process used to prepare and finish the wood. Some companies that use reclaimed or recovered wood use environmentally damaging methods of turning that wood into your floors. Ask questions about the finishing process to make sure the green wood stays green.