Balsa is the common name for a fast-growing, tropical American tree, Ochroma pyramidale (synonym O. lagopus), characterized by soft and light wood. The name also is used for the wood of this species, which is commercially important, and despite being very soft, is classified as a hardwood—the softest commercial hardwood.

As a light, soft, and buoyant wood, with good insulating properties, balsa provides innumerable uses, from use in life preservers and floats, to model building, to fishing lures, to insulation use in refrigerators. Ecologically, balsa trees provide food for bats in a symbiotic relationship tied to pollination of the trees.

http://www.newworldencyclopedia.org/entry/Balsa

http://en.wikipedia.org/wiki/Ochroma_pyramidale