Someone in GameFAQs asked this question as to what exactly it is that makes shffl attacks special.
For those who don't know, shffl stands for the following:
sh = short hop
ff = fast fall
l = l-cancel
Pronounced "shuffle."
If anyone had any sort of question about shffl attacks, then I'll post my own reasons on why shffl attacks are so effective at racking damage and harassing your opponent.
First of all, shffl attacks are the quickest and the most efficient way of attacking your enemy. In a simple way of explaining how shffl attacks work, it allows a character to use aerial attacks as if they were on the ground. But one would ask, why not just use ground attacks?
Because ground attacks are limited, usually laggier, and forces you to stand still while using them, as opposed to shffl attacks, where you can control and move in the direction you want to attack, and L-cancel drastically lets your character move faster after each attack. The closest thing you have for a ground attack where you can still move is a dash attack, and even then, it's inferior to a shffl attack in that you have to deal with the lag at the end of the attack and is easily counterable.
As for the shffl itself, what makes it so effective? Let's take it apart piece by piece here.
First, what you want to do is a short hop, which helps you use your aerial attack at a range where he can still make contact against your opponent on the ground.
Your attack comes next. Your options are basically all the aerial attacks that you can use: N-air, F-Air, B-air, U-Air, and D-air. Each of the first letters correspond to the direction of your attack, with N being neutral. Generally, you'd want to use your attack right when you hop off the ground, but sometimes, it's situational. If I hop and I find myself initially out of range, I keep moving forward and attack when I reach my opponent, skipping the fastfall all together. Anyway, let's not complicate things: learn to attack right from the short hop.
Then you'd want to fast fall when you're at the peak or while you're descending. The point of the fast fall is so you can land on the ground as quickly as you can, which is where L-cancel comes in.
Finally, the L-cancel, which reduces the lag from your landing animation. The easiest way to tell the difference is with Link's D-Air. Without L-cancel, Link usually takes a painful long time to take his sword out of the ground. With it, you'll see the quickened pace. In competitive/advanced play, when you learn what you want to do next and instinctly train yourself to use that action after you L-cancel, you'll realize how much of a difference it'll make on your gameplay when you do or don't L-cancel.
For example, if you're using Fox and plan to shine after L-canceling, you'll find that not being able to L-cancel leaves you open for a few good frames, long enough for someone to grab you or jump out of the way before you can use the shine.
So what do we get without any of these components? Without a short hop, you won't be able to attack with your aerials close to the ground as you want to. Without a fast fall, you'll find yourself hanging in the air longer then you want to, especially since you'd want to make your next move right away. Without an L-cancel, you'll find that you're not moving as quickly as you should be, and the few frames that it takes for the animation to end could cost you the match.
Practice with characters that are easier to short hop with. Bowser and Ganondorf normally make good choices. Practice the essentials. Use N-air, F-air, and B-air first. Next, try attempting a D-air and then the U-air, which are normally the harder of the 5 aerial attacks to shffl. If you want, you can learn to shffl your attacks using the C-stick. I'll admit that I've been using C-Stick up to use a shffl U-air with Fox, but quite recently, I'm getting better at using it with just the control stick.
Eventually, you'll want to move on step by step to characters that are more difficult to short hop with, like Link or Mario. Then move on to Fox and Falco. Etc. etc.
Also, study each character and realize that shffl attacks aren't useful for every single character out there, or that there cn be some exceptions. For example, rarely do I see a Shiek use a shffl attack, but Fox uses it quite constantly. A Samus player normally wouldn't shffl unless it was out of a bomb jump. Some players don't fast fall Captain Falcon's N-air in a shffl to allow both kicks to hit. Learn these things about every individual character.
Any questions? Hope that helps out a bit.