A ParticleSystem is responsible for being the glue between the different components and it's also the one doing all the actual simulation.
It has 3 different component fields:
The ParticleBehaviour is an array of different ParticleBehaviours, the array size is 8 by default, but can be changed from within the engine using the ParticleBehaviourCount define.
There are several different fields you can change on the ParticleSystem, most of them you'll know from the old particlesystem like PartLifeTime. The emission fields has changed though, instead of setting the time in ms between particles being emitted, you know set the ParticlesPerSecond so you are not limited on how many ParticlesPerSecond you want to emit, and it's overall more intuitive.