According to The American Heritage Dictionary of the English Language:

Par·ti·cle - noun:
1. A very small piece or part; a tiny portion or speck.
2. A very small or the smallest possible amount, trace, or degree.
3. (physics) A body whose spatial extent and internal motion and structure, if any, are irrelevant in a specific problem.

     The 3rd defenition is the one that pertains here. A particle is an object that has mass, position, and  velocity, and responds to forces, but has no spatial extent. In other words, particles are as small as specks of dust floating in the air, and are also affected by such forces as gravity, inertia, and other physical forces. This makes a particle system an excellent way to simulate the movement of fluid systems such as air, water, and fire, as well as any type of randomized movement of a group of objects. In the picture to the left, the particles are displayed as little plus signs (called "ticks") to make it easy to see their motion. The ticks are drawn with a hairline stroke in Flash, so that they are small but visible no matter how far you zoom in or out. You can also view them as dots, triangles, squares, spheres, or any external swf or jpeg. Being objects with no actual spatial extent means that we can load any piece of artwork into the particle system, and every particle will be created as a copy of that artwork.

    Now for the real power pf particles. What makes particles useful for achieving elemental  effects is by placing instanced objects on each particle. So, for example, if you have a short animated movie clip of a bird flapping it's wings, you can have each particle be a copy of that animation, so the end result looks like a flock of birds, all flapping their wings. By using some basic graphic design techniques, you can build artwork to be copied as particles that, when semi-opaque and overlaying one another, give the effect of clouds, smoke, fire, and other elemental effects. The fire pot to the right is an example of how a single small png with alpha transparency can be used to create realistic-looking fire effects. This is also one of the presets described in the Tutorials section.

    With the ElementalFX particle system I had to make a few compromises to ensure that the math required to calculate the position and properties of each particle doesn't cause your Flash movie to run slowly. So this is not an honest-to-goodness true physics 3D particle system. To see an example of a true physics 3D particle system at work, watch the movie Armageddon. Not only did those two space shuttles not exist (they were computer generated and added to the footage of the lauch pad digitally), but the booster rockets and the smoke trail coming out of the back of them did not exist either. Those exhaust trails were created with one or more particle systems (using Discreet Combustion and Animatek Afterburn), and the end result was an absolutely realistic exhaust trail behind the rockets. Now, you must understand that the particle system used to produce the exhaust trails in Armageddon are pretty much the top of the line software running on the top of the line hardware, and it still took several weeks to calculate all of the particle positions. So, doing something like that in real time in Flash is just out of the question. On a smaller scale, however, particles can still be very useful for achieving the same kinds of effects, you just have to take into account the processor speed and amount of ram that the average user has, and the performance capabilities of Flash. With ElementalFX I have developed a small-scale particle system capable of using any external swf or jpeg as a particle.

    The end result, as you can see, is a stunningly realistic elemental effect. Particle systems can be  confusing and complicated, but in the end, the results are worth it. ElementalFX can even be used in layered effects. By placing two or more particle systems on top of one another, you can create endless amazing effects with almost no addition to your file size. While the 1.0 component does add about 70k to your file size when you initially drop it on the stage, it doesn't add any more to the file regardless of how many different particle systems you use in your movie, and with the new version for Flash MX 2004, the footprint is under 10k. Additionally, the new version also supports particle coloring (for vectored particle artwork) and particle tinting (for rastered particle artwork), both of which can be animated over time (as seen in the spray sample near the top-left of this page).