I fear that I have not explained particles well enough in the last post, so I am adding another today. The beauty of particles, is that you take one image, a small image, and replicate it in some way. Maybe you replicate it into many places on a screen. That is one particle system. A bunch of dots on the screen is one representation of a particle system. The system is accurate at that moment, but the next second, it will look different. Why? The particles moved. Hence, the term "particle system" with system being the more important term here.
The beauty of a particle system is the ability to describe how long one particle will behave. Behave is a bit obscure, but to programmers this mean describing what will happen to this particle in its lifetime. It is an example of the worse kind of interrogation:
When are you born? What is your life span? What color are you? What time do you change colors? Whats your new color? Are you changing sizes at any time? Can we push you over here?
The system is what describes what the particle does. Throw in all the variables that answer all of these questions and you have a bunch of seemingly similar objects behaving in a remarkably way. The objects move in a manner that is similar, but unique to each objects manner.
That is confusing until you see one object behave according to a system of rules. Just layer another object behavior on the same area, and you begin to see the complex system described. Essentially, each object has their own agenda and will follow it's own system to complete the agenda. Particle systems are like that.
"Every one go left, but grow and shrink as you do it."
Take 30 objects that follow the command and you get an interesting jog to the left.
Watching todays local celebration of the Seahawks bringing home the Super Bowl trophy reminded me of a particle system. (Not to knock on the celebration. I am a local Seahawks fan and really was moved by the celebration and the team's excellent response to the fans.) Everyone is there and moving around within a manner consistent with their individual system. Looking at each individual moving around you would not notice any specific pattern. On the other hand, pulling the camera back you can see a lot of people all enjoying the celebration. They are moving all over the place and create a very unique view of fans enjoying a celebration. Take a picture a few hours later and the the image would look very different. It is all relative to time and the location and the clothing and the light of the day and... you get the point. Each person is doing their own thing, but the picture of all the elements is surprisingly complex.
Much like a particle system.