Wreckage is found in one location, debris is spread or scattered over an area. Otherwise they both mean "the stuff left over after something has been destroyed". In astronomy, debris (never "wreckage") is the material shed by a comet, or the fragments resulting from a collision.

Wreckage usually refers to large chunks of material or even complete (but destroyed) objects and tends to be used more for moving objects, like airplane and car crashes. "It took 15 minutes to remove the survivors from the crash wreckage." Debris usually refers to smaller pieces, scraps and dust and is often used to refer to buildings, perhaps destroyed by tornadoes and other natural disasters, or terrorist attacks. The World Trade Center buildings' collapse spread debris over all of downtown Manhattan." However, sometimes you can use both to describe one event. "The bulk of the airplane wreckage was found in the forest, but a lot of debris was spread out in the surrounding fields."

You would not say "We need to move the wreckage out of the way before we continue walking." you would use a word like rubble or debris. Debris is the scattered remains of something and wreckage is something which has been severely damaged or destroyed.