Otherwise known as the physical plane. It is the place where creatures of all types live out their lives. You cannot see or hear anything in any other plane while in the material plane.
It is in this plane that the world of Mysteria exists.

Ethereal Plane

Certain types of spellcasters have the ability to travel to this place. Some can even take others there. Some creatures live in the ethereal plane, returning to the Material plane to feed.

Understanding the Ethereal plane is almost like understanding the afterlife. An ethereal creature is incorporeal, and invisible to those on the material plane. Capable of moving in any direction, even up or down.
As an incorporeal creature you can move through solid objects, including living creatures. An ethereal creature can see and hear the material plane, but everything looks blurry and insubstantial.

An ethereal creature cannot attack a material creature, and spells cast while ethereal only affect things on the ethereal plane. Certain creatures have abilities that work on the ethereal plane.
By contrast, treat other ethereal creatures and ethereal objects as if they had become material.

If you end the spell or become material while inside a material object (such as a solid wall), you are shunted off to the nearest open space and take 1d6 points of damage.

Traveling through the ethereal plane also takes quite a bit out of a person after they return to the material plane. Once returned to the material plane a person loses 10% from their hit and evade for 1d6 rounds of recovery.

Celestial Plane

Called "heaven" by some. The term "celestial" refers to many beings who live on the plane of Good. Celestial practically drip with goodness, every fiber of their bodies and souls is suffused with it.
It is unknown if anyone from the material plane has ever made it to the Celestial plane unless they had died. Although some creatures return to the material plane from the Celestial plane as a celestial warrior. Examples of some are: Lantern Archons, Hound Archons, Avorals, Ghaeles, Trumpet Archons, Astral Devas, Planetars, and Solars.

Infernal Plane

Sometimes refered to as "The Abyss", "The Realm of Unmitigated Chaos", or even "Hell". The infernal plane is a place of true darkness and evil.
Like the celestial plane, it is unknown if anyone has ever ventured to the infernal plane without dying first. Also like the celestial plane, those that have died and traveled to the infernal place may resurface in the material plane as something truely evil. For instance: Tanar'ris, Dretchs, Quasits, Succubus, Bebiliths, Retrievers, and Vrocks. All mentioned are species of demons.