My first encounter with the terms "flotsam" and "jetsam" was as an enraptured kid watching the wonder that was "The Little Mermaid". Flotsam and Jetsam were the names of Ursula's two pet eels (who dutifully fulfilled her evil tasks), but it wasn't until much, much later (till now, in fact), that I learned the exact definitions of these two related, yet distinct, terms.
Before digging into flotsam and jetsam, we must first look at another word: "jettison". Jettison came to us via the Anglo-French noun, geteson, the "action of throwing", and ultimately from the Latin verb, jactare, "to throw". Jettison, once a noun meaning "a voluntary sacrifice of cargo to lighten a ship's load in distress" entered English in the 15th century. The verb has been with us since the 19th century. Jettison these days has separated from its ship association, and can simply mean "to discard".
From jettison we get jetsam. Its original form, jetson, a syncopated form of jetteson, was soon perverted to jetsom (the OED notes this may have perhaps come by association with native words ending in -some). Jetsam is a noun denoting the goods jettisoned from a ship.
These days flotsam and jetsam are almost always paired; even pop culture has seized onto this phrase, as we saw with Ursula's eels. Flotsam more specifially refers to wreckage floating in the sea, and is dervied from Old French, floter, "to float".
Who knew ship wreckage had such specific names? Through looking at language, we can learn so much about the history of our culture -- these example in particular indicate our sea-faring nature, and the journeys and pillages that were all a part of founding the New World.