The distinction is important (and not just so you can impress friends during trivia games) because, under maritime law, the original owner can reclaim flotsam, whereas it's generally a case of "finders, keepers" in the case of discovered jetsam.
Flotsam and jetsam are terms that describe two types of marine debris associated with vessels. Flotsam is defined as debris in the water that was not deliberately thrown overboard, often as a result from a shipwreck or accident. Jetsam describes debris that was deliberately thrown overboard by a crew of a ship in distress, most often to lighten the ship's load. The word flotsam derives from the French word floter, to float. Jetsam is a shortened word for jettison.
NOAA's Marine Debris Program aims to mitigate problems caused by the presence of flotsam and jetsam in our national waterways. Boaters should always practice responsible disposal of all waste materials, including fishing gear, for example, which can "entangle, injure, maim, and drown marine wildlife and damage property."
So, now you know, friends - mind your flotsam and jetsam!