Fencing is a sport derived from western swordfighting. In its modern form, it is like a linear, low-impact form of boxing. Electronics are now used to detect hits (or "touches") and the game is more about strategy and skill than strength.
It has three main forms - foil (only hit opponent's torso, all hits made with point), épée (hit anywhere, can only use point) and sabre (hit torso, arms and head using any part of the blade inc. point). Foil and sabre have a system called "right of way" for decided who gets the points. In épée, if you hit, you score a point.
It is "explosive" - meaning most of the actions done will be rapid changes of pace (as opposed to say, "endurance" activities like running marathons or satisfying one's girlfriend).
A: Did you see the Olympic fencing?
B: Some - it's the only one where you can't see how fit the athletes are because they're wearing three layers of kit and metal blocks on their heads.
A: You're not a fencer, are you?