There are many different planes of existence, “touching” some of the other planes under special circumstances, but mostly existing separately from each other.
Material Plane: The everyday world as we know it: Gaia, the Sun, the Moon, Space and the Stars.
Elemental Planes: The four Elemental Planes are the Plane of Air, the Plane of Earth, the Plane of Fire, and the Plane of Water—it is from these planes that the creatures known as elementals hail, yet they house many other strange denizens as well, such as the genie races and the mischievous mephits. In situations with high energy discharges, like thunderstorms, forest fires or earthquakes, denizens of the Elemental Planes can cross over to the Material Plane. Also they can be summoned by spells like Elemental Swarm.
Astral Plane: A silvery void that connects the Material and Elemental Planes to the Celestial Planes, the Astral Plane is the medium through which the souls of the departed travel to the afterlife. A traveler in the Astral Plane sees the plane as a vast and misty space. Powerful spell-casters utilize the Astral Plane for a tiny fraction of a second when they Teleport, or they can use it to travel between planes with spells like Astral Projection.
Clerics and Shamans can send their spirit to this plane when they meditate.
Celestial Planes: It is to these Planes that the souls of the dead travel, and it is in these planes the gods have their dwelling places. Without a Divine invitation they are closed to all beings from the other planes.
Ethereal Plane: The Ethereal Plane is a ghostly realm that exists as a buffer between the Nether Plane and the other Planes. A traveler in the Ethereal plane experiences the real world as if the world were an insubstantial ghost, and can move through solid objects without being seen in the real world. Strange creatures dwell in the Ethereal Plane, as well as ghosts and dreams, many of which can sometimes extend their influence into the real world in mysterious and terrifying ways. Powerful spell-casters utilize the Ethereal Plane with spells like Blink, Etherealness, and Ethereal Jaunt.
Nether Plane: The eerie and deadly Nether Plane is a grim, colorless place, one infused with negative energy and serving as home for the chaos and evil of the Abyss, the regimented lawful evil of Hell, strange monsters like undead shadows and worse. Powerful spell-casters utilize the Nether Plane to swiftly travel immense distances on the Material Plane with Shadow Walk, or draw upon the mutable essence of the Nether Plane to create quasi-real effects and creatures with spells like Shadow Evocation or Shades. The denizens of the Nether Plane have no power of their own to reach the other planes. All their power comes from the negative emotions of creatures of the Material Plane. Wittingly or unwittingly experienced in dreams, or through evil rites and sacrifices, Evil grows from greed, hatred, contempt and indifference. Demons from the Nether Plane feed on those negative emotions and they try to tempt those who have them to ever greater evil and ultimately to worship them. Thus giving the demons power over them and access to the Material Plane.