Jetsam is debris floating around the water that has been dumped from a sinking ship. Sometimes beachcombers find jetsam washed up on shore.
You often see the phrase "flotsam and jetsam" because both words refer to wreckage from a ship, but how the stuff got there is the difference. Flotsam is from a shipwreck, and jetsam is thrown off the ship to prevent it from sinking. People often use jetsam to mean any discarded objects, not just from the ocean.