The Physical plane can be imagined as a low bass sound. So low that it can be felt, as well as heard, the mental body overlapping the physical plane seeing it clearly as it is so well defined and quantifiable by our five physical senses.

With the Physical Plane represented as a low bass, the Astral Plane can be thought of as a something beyond the hearing of humans; with the mental body needing to adjust it's perception far above what is normally necessary for interaction on the physical plane.

The astral plane can be thought of as having "layers". These layers are defined by their vibrational qualities, or their frequencies. Like a radio an individual can learn to "tune in" to the different frequencies.

The astral plane is divided into an infinite number of frequencies, which can be tapped into by the experienced practitioner for a variety of reasons. These different frequencies of the astral plane are also commonly referred to as planes. A simple analogy would be an AM Radio. The individual "tunes" in a specific frequency (That which he or she wishes to work with), and then increases or decreases the "volume" to determine the energy level that they want to work with. It is commonly believed that the Astral Planes are organized as a hierarchy with "Lords" being at the top of this ladder.

One of the aspects of the Astral Plane is that through it, with experience an individual can develop a variety of talents. This includes but is not limited to clairvoyance, clairaudience, communicating with other people, contacting spirits and entities, and utilizing the Akashic records. The method for each of these talents varies, as there is no single way of learning how to operate on the Astral Planes.

There are Elemental Planes, which are the homes to the various forms of elementals. Earth (gnomes), Air (sylphs), Water (undines), Fire (salamanders), are some of the more common forms that are taken by elementals.

There are Planetary Intelligences that operate on the Astral Plane. Each with it's own Hierarchy, and vibrational quality. These planetary intelligences as they are referred, are entities that are associated with planetary influences, and energies. They are generally archetypal in nature.

Other denizens of the Astral Plane include but are not limited to, demons, angels, poltergeists, larvae, and deities, each having their own sphere of influence, and their own properties. Traditional pantheonic deities are also accessible via the astral plane. The techniques of contacting these entities vary from tradition to tradition.

I would like to stress that contacting entities and beings from the astral planes is not something that should be undertaken lightly. A great deal of study is required into the methods of engagement of particular entities. One must be aware that casually "playing" with beings that are encountered on the astral plane can, and usually is, dangerous. Appearances can be and often are deceiving, there is no substitute for being prepared, and knowing exactly what it is you are doing and what you are dealing with.

Further Reading

Practice of Magical Evocation