There are various planes of being, planes of consciousness, worlds, cosmic ethers, levels of spiritual evolution, etc. Almost all esoteric schools agree that there are 7 such planes and each one is divided into several levels. The planes don’t necessarily exist in a different place, but in a different state of awareness – a different layer of the fabric of time and space. They exist externally, as in other worlds of being, but also internally as in planes of consciousness. They are coexistent with our physical world but are another dimension of reality. It is possible to move within and around the different planes without moving the physical body. So, for example, while we sleep we may unconsciously pass into the Astral Plane. The planes form a part of us (or we are a part of them) and as such are subjective states; but we project this subjective world outside of us and thus perceive it as objective.
The 7 planes are usually described and labelled as Divine, Monadic, Spiritual, Intuitional, Mental(Causal) Emotional (Astral) and Physical (Material). We experience reality within the Physical / Material Plane. The Emotional or Astral Plane is the next beyond us and the easiest for us to access. It is a world of emotions, desires and passions. The Astral Plane itself is traditionally divided into 7 subtle levels. The lowest level is closest to the Physical world. Movement up the levels requires more and more spiritual evolvement. The Astral Plane is inhabited by a variety of non-human entities. The more spiritually evolved beings occupy the higher levels (closer to the Mental/Causal Plane and beyond it) while the grosser ones occupy the lower levels (nearer the Physical/Material Plane). Most people that enter the Astral Plane, whether through sleep, an intentional journey (astral projection), or even in death, are interacting with the lower level beings. The entities are known in various cultures by names like nature spirits, elementals, demons, angels, fairies, goblins, sprites, elves, and humanoid spirits and so on – and these beings also often intrude on the Physical Plane. The psychologist Carl Jung regularly visited the Astral Plane and encountered a human like spirit named Philemon, with whom he had many interesting discussions. Jung claimed that meeting Philemon proved the objective existence of the psychic world. The Astral Plane is considered to be the first Plane we pass through after death. Depending on our own spiritual development we may ascend to higher levels and perhaps pass over to the next higher planes.
The 5 planes beyond the Astral require a more evolved spiritual nature to enter. In the historical record, the highest plane a human has experienced while still living was the Spiritual Plane. The person who experienced this was St. Paul and he described it as the Third Heaven. Most mystics, however, have never gone beyond the Intentional Plane. The Mental Plane is the more common realm for visiting. It is believed that the Mental Plane is where humans remain between incarnations and is sometimes called the Heaven plane. True clairvoyance is also practiced through this Plane as life in its entirety can be perceived from here.
Most mediums and channelers, if they make any real contacts, are communicating with beings on the Astral Plane. Humans who are highly developed spiritually move higher up the levels and do not readily interact with people still on the Physical Plane. Those who are not highly developed remain longer on the Astral Plane, at the lower levels. It is easier for these spirits to communicate with the living. Thus, it is mostly the lesser developed beings that transmit messages to the Physical Plane.