Isn't that what we're all asking? Put simply, D-Style is a strategy in Super Smash Bros. Melee, using (some people say overusing) Marth's counter move. The idea is to use the move as much as possible when the enemy attacks, rather than shielding, dodging, or using an attack move. Although most effectively used for Marth, it can also be used for Roy. Peach has a counter move too, but don't try using it or you'll get owned ;-)
I want to use D-Style... but HOW??
It's easy! Marth's counter move can parry nearly any attack. If an attack hits him within a short period while he's using counter, he will not take damage, but will strike the enemy, dealing some damage. If you want to use D-Style, you have to be able to predict when the enemy will use an attack, and when they do, you use counter right in their face. Sound easy? It is. But remember, you can't win on countering alone. Use D-Style to turn the enemy's moves against them and drop their defenses, giving you a wide open chance to attack. It works great in the middle of an enemy combo.
Why D-Style works:
D-Style works because the enemy usually doesn't expect you to counter them. It comes as a surprise. If you're any good at it, you will be able to counter any move you want, making you practically untouchable. The other player will often get frustrated and thus their playing ability will drop, allowing you to finish them off. With Marth's superior range used wisely with D-Style, nearly any opponent of equal skill will have a tough time defeating you.
Does D-Style have a weakness?
Like any strategy, D-Style has weaknesses. The main weakness is that if the opponent doesn't attack you at the very beginning of your counter, you are left vulnerable for a short period where you are immobile. It takes practice, but with time, you won't have to worry much about that because they'll hit you every time you counter (much to their dismay)!
The other weakness you'll encounter with D-Style is that it is completely useless against grabs. If the enemy grabs you, you cannot counter them. In fact, you can't do anything at all. That is why you want to avoid this situation at all costs. Remember, the enemy can't grab you while they're in the air, right? You can also use their foolishness to your advantage. If you see them start to use an attack, such as a charging forward smash... that's your chance to counter.
A Summary:
D-Style is just using Marth's (or Roy's) counter as an offensive strategy. Use it when the enemy is going to attack, in the air, by the edge, or whatever. I'm not saying D-Style is the ultimate strategy, because it's not. But if you use it in conjuction with any other strategy, your playing is sure to improve. It works best on players not expecting the D-Style to come, so use it to your advantage. Just practice to get the timing right. It also helps to know the other player's style of playing to better help you predict when they will attack. But most opponents will play the same... attack whenever the enemy is near.
Oh yeah, and don't use D-Style against a computer player. It won't work. You shouldn't be playing computer players if you want to get better anyway.