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 ...