A particle system is a method used to create effects such as fire, smoke, or snow. The technique involves rendering a lot of small particles at once. A particle is a small picture. It's hard to define what it should represent. In the case of snow, it should be a single snowflake. When creating smoke or fire a small fire ball or a puff of smoke would be a good start.
A particle system usually creates a particle at a given point using initial parameters such as scale, opacity, and rotation. A set of modifiers is also created and each particle is modified by all modifiers over time. A particle system also defines the lifespan of a particle and how many particles are to be spawned.
Making particle systems look good is an art and there is no single way to do it. We are going to see how we can create two completely different effects using the same picture. Our game doesn't really need particle systems and so we will add one of these effects to the menu scene just for illustration...