This snowboard shape is better to ride and land in switch and to rotate in general. Twin shapes are mostly used in freestyle / jump / pipe / jib snowboards. Found in all the snowboards described as "true twins", in some described as "directional twins", and maybe even in some described as "directional".
On a snowboard with a twin shape, the shape in both sides (from tips to center) is equal.
Directional shapes may excel in going forward without the compromise of still having to ride great in switch, even though they may be able to ride switch, that surly wont be their strongest skill. They're usually made for specific end results like being better at carving / riding fast, riding powder effortlessly... Directional shapes are mostly used in snowboards made for powder / freeride / speed (carving) / all mountain. Found in snowboards described as "directional" and "directional twin".
On a snowboard with a directional shape, the shape is not the same on both sides. One or more of the following things occur:
(note: the most directional snowboard possible would have a directional shape, a directional flexibility, a directional profile and set back inserts, and would definitly be called "directional". A directional board that misses one or more of those directional specs. may be called "directional", "directional twin" or "twin", just not "true twin")