There are tons of techniques to visualize your idea… and plenty of room for more invention.

Lemme bring you a few of the most common ones closer:

-A favorite seems to be tween-based animations, the kind we see in movies like Return of Ganondorf. This breaks the body into several layers of body parts and uses tweens to animate characters.
ADVANTAGES: Relatively easy, one high quality image (IE: a torso with very complex, elaborate armor) can be developed and used throughout the entirity of the movie.
DISADVANTAGES: Less dynamic. Characters may tend to stay at fixed angles to the camera. Foreshortening will require variations of limbs if you want to do it. Doesn’t substantially give you better experience as an animator.

-Frame by Frame.I use it most of the time. This is an animation technique means each frame is redrawn. Sometimes, frames may be recycled, but each frame of a characters animation is freshly drawn. The “onion skin” feature of Flash MX is a very powerful tool for this type of animation.
ADVANTAGES: This is the most absolutely dynamic animating style humanly possible that I am aware of. You literally control every single frame of the movie, and are able to make fluid, realistic, dynamic movements. Also, the effort put into doing this is extremely beneficial on an artistic level, as it takes a lot of drawing to do.
DISADVANTAGES: Extremely time consuming. Results will be pretty weird looking without good drawing skills and a wacom tablet. Character designs in FBF are harder to edit after you’ve begun than in tween-based.

-Some people will do frame by frame, but only redraw the portions of the body, etc. that require changing. So if your character is throwing a punch, You’d only animate the arm, maybe the twisting of the torso, frame by frame, but leave the feet in one place.
ADVANTAGES: Not as time consuming as true FBF. More realistic than tween-based.
DISADVANTAGES: Not as dynamic as true FBF. Still harder to do than tween-based. It’s very important that you stay consistant between frames, as some will become static images when not in motion.

-’Clip’ style. Make a series of drawings in a graphic program (IE: Photoshop) and put them into graphics, movie clips, etc. in Flash. Replace animations with other ones when you want that animation to stop. I know that “Galactic Stampede” that was daily feature on newgrounds.com a month or so ago used this style, and it was pretty impressive.
ADVANTAGES: Maximumizes quality. Allows for cool effects like Guassian blurs that you normally can’t do in Flash MX. Very dynamic. Looks extremely amazing when used right, and can knock people off their feet.
DISADVANTAGES: Using this style tends to limit the number of actions a character can do or interact with the environment. Authors will tend to stick to ‘generic’ or ‘multi-purpose’ animation, much like sprites. Since character is not drawn directly into Flash MX, this mode is hardest to edit after you’ve begun. This style also requires a lot of work, somewhere close to if not equal to Frame by Frame, since the bar for your image quality will be higher.

Those are the ones I’m aware of, but one point I think I should bring up:

A lot of authors use more than one technique. Take Alanthebox´s “how it will end” for example,(google it) He used alot of different styles and animation techniques.

I’ve used tween based before, but for something like a magical spell, I’ll animate the fiery blast’s movement frame-by-frame to get the most fluid, impressive look I can.

Best of all — come up with a new one! And when you do, be sure to share it. Techniques are a great thing!