Emitters generate particles within given volume and provided direction spread. Particles themselves have no visual representation and must be passed to appearance nodes to be seen.
Each particle generated has several initial properties: position, orientation, motion vector and lifespan. Depending on hierarchy of node instances defined in effect block particles may or may not inherit transformation matrix of its emitter (or a completely different node).
Appearances visualize particles that are generated by emitters.
Fields can affect position, orientation and motion vector of particles generated by emitters.