If you have ever tried to explain it to your friends, you know it's pretty difficult isn't it? It's slighty easier if you try to explain "what is a downwinder" to your friends who paddle. I think part of the problem is the word itself. "Downwind" is the direction of travel and "er" describes the action but does not convey what you are actually doing. Here's my attempt at explaining a downwinder in as few words as possible:
A downwinder is a fast, one-way paddle in a following sea with the goal of getting your board to plane (surf) down the face of a wind wave (bump) by adjusting heading, stroke cadence, and board trim. The ultimate goal is to catch long glides and link them together so that your board remains on a plane for as long as possible.
I guess that is decent but still does nothing to describe the way it makes you feel. Close your eyes and imagine being a Norse god thundering through the clouds with the spray on your face, the sun on your back, paddle raised triumphantly into the air after acing an exam on your birthday. Yeah, like that only better!