Particle systems are objects that generate non-editable sub-objects, called particles, for the purpose of simulating snow, rain, dust, and so on.
The particle system object generates the particles over time. You use particle systems primarily in animations.
3ds Max provides several built-in particle systems, including Spray and Snow. 3ds Max also offers an event-driven particle system called Particle Flow. Your configuration might have other plug-in particle systems installed.
The Deflector, Gravity, and Wind space warps are for use with particle systems.