Both refer to debris found in the ocean, but they differ based on how/why they got there.
Flotsam
This accounts for the majority of the rubbish you find in our oceans. Flotsam refers to those articles which accidentally make it into the ocean, either by blowing off boats or left on beaches. This is why its so important to either secure items when traveling on boats, or pickup after yourself after a day on the beach.
Jetsam
Thrown out deliberately, e.g. when trying to lighten a ship stuck stranded on a reef. I remember this, as a pilot can jettison himself from an aircraft when pulling his emergency shoot.
