A particle system consists of a group of discrete particles which change over time. Each particle has its own attributes controlling size, position, velocity, etc. They can generate new attributes depending on their age, or they may die. Assigning values discretely to each particle enables realistic modeling of systems involving turbulence such as: smoke, wind, fire, dust, and hair.
You can use any point or set of points as the basis for the particles in a particle system. Grid SOPs or Sphere SOPs are often employed for this purpose. You can set the particle system into motion by applying a Particle SOP. You can then subsequently influence the particle system with a Force SOP.