What is it? This is the plane on which the game largely takes place. The physical world, where people live, work and play.
Who can access it? Everyone, although those who exist primarily in The Veil have a hard time doing so without assistance.
What is it? The thin curtain one cross after life in death, where non-departed spirits dwell. It seems/feels/looks different to each spirit.
Who can access it? Spirits, mediums, and those with similar abilities can see through the Veil and speak to other spirits through it. There are those who can pull spirits through for the sake of possession, communication, hauntings, etc. Occasionally a spirit may break away from the control of these summoners and roam the Material Plane, restless.
Shadow Plane / The Void
What is it? A dark plane inhabited by mysterious shadow versions of things from the Material Plane, and/or Lovecraftian beings. Exposure to this plane has a tendency to induce madness, because of the absolute dark and the twisted nature of what dwells in such a land. The plane itself is covetous, it hungers for the rare lifeforce able to pierce it and will attempt to keep that life near, whispering and bading frequent return visits. Rumors abound that people are spiritually tainted by exposure, which can likewise lead to physical and psychological decay.
Who can access it? Very limited access! One must either be born with the ability to magically break through to this plane, or be aided by one who is able to. There is tremendous risk in doing so, as interaction with this plane will stain the trespasser, both in flesh and mind. Visitors become paranoid and drawn to return time and again. Most symptoms fade with time away, but do you really wish to risk it?
The Dark Realm
What is it? Essentially this is Cirque Nocturne's version of Hell or The Underworld. Natural home to demons and the demonic. The place where spirits who are “damned” end up once passed beyond The Veil. Not a nice place, generally speaking!
Who can access it? Pure demons may pass back and forth between Dark Realm and Material Plane. Those with demonic magic may summon things from this realm, and of course, the "damned" dead.
The Mist & Avalon
What is it? The Mist is the barrier that conceals the Fae Realm, otherwise known as Avalon. Avalon is the font of natural magic from which fairies, unicorns, pixies, brownies, etc spring. There are territories / courts scattered throughout, with their own inhabitants, squabbles and dramas.
Who can access it? Fae royalty are able to pass through The Mist as they please, as they are quite old and soaked in the essence of Avalon - if not an aspect of it, themselves. Others may pierce The Mist, through birth or gift, but it is rare, and the ruler of the breached territory is always instantly aware of what has been done and where.
The Mirror Plane
What is it? When the first magic mirror was used to view or travel distances, The Mirror Plane was born. It grew from there, and has since become a place where any who travel via mirror pass through. Likewise, anyone who has ever been reflected has a double living in this plane. The more you travel through, the more aware of you this Other is. Be wary that they do not reach the exit before you, as it would trap you in the Mirror Plane in their stead - allowing the Other to live your life.
Who can access it? Any who travel by mirror magic, or an Other who has successfully escaped. Likewise, those who have Mirror magic can trap an entity in the Mirror Plane - either by their magic or by delaying them until the Other can make it to the exit.