The beginning point which it is essential to make clear in describing this astral plane is its downright reality. I am utilizing the word in its plain, day-to-day sense, and I mean by it that the objects and inhabitants of the astral plane are genuine in precisely the same way as our own bodies, our pieces of furniture, our homes are real. 
They'll no more endure for ever than will objects on the physical plane, but they're nevertheless realities from our viewpoint while they last—realities which we can't afford to ignore simply because the majority of humanity is as yet unconscious, of their existence.
The Background
The names commonly dedicated to these planes, taking them in order of materiality, rising from the heavier to the finer, are the physical, the astral, the mental or devachanic, the buddhic, and the nirvanic. Greater than this last are 2 others, but they are so far above our present power of concept that for the moment they might be left out of consideration. It ought to be understood that the matter of each of these planes differs from that of the one below it in the same way as, though to a much higher degree than, vapor differs from solid matter.
The astral region which I'm to try to describe is the second of these planes of nature—the next above (or inside) that physical world with which we're all familiar. It has frequently been called the realm of illusion—not that it's itself any more elusive than the physical world, but, as of the extreme un-reliableness of the impressions returned from it by the untrained seer. This is to be accounted for chiefly by two remarkable features of the astral world—first, that a lot of its inhabitants have a fantastic power of changing their forms, and also of casting practically inexhaustible glamour over those with whom they choose to sport; and second, that sight on that plane is a faculty really dissimilar from and much more extended than physical vision. An object is viewed, as it were, from all sides at one time, the interior of a solid being as plainly open to the view as the exterior; it's therefore obvious that an ignorant visitor to this new world might well find considerable trouble in understanding what he truly does see, and still more in transforming his vision into the very inadequate language of ordinary speech.
He has to learn not only to see properly but to transform the memory of what he has seen precisely from one plane to the other; and to assist him in this he's trained to carry his consciousness without break from the physical plane to the astral or devachanic and back once more, for till that may be done there's always a possibility that his recollections might be partially lost or distorted during the blank time interval which separates his periods of awareness on the assorted planes.
The first introduction to this noteworthy region comes to individuals in assorted ways. A few only once in their whole lives under some strange influence become sensitive enough to recognize the presence of one of its inhabitants, and maybe, because the experience doesn't repeat itself, they might come in time to trust that on that occasion they must have been the victims of hallucination: other people find themselves with increasing frequency seeing and hearing something to which those around them are blind and deaf; other people again—and maybe this is the commonest experience of all—start to recollect with greater and greater clarity that which they've seen or heard on that other plane during sleep.