Adurant

According to elven scholars, the planes are divided into two distinct entities - the Astral and Ethereal planes - which overlap to form what is known as the Material plane. Both other planes are homes to spirit creatures, which occasionally manage to venture and find forms on the Material plane. These occurrences cause hauntings, or take on powerful demigod-like forms. The difference between the two planes is rather minor, but it is believed that the Astral plane is the realm of the mind and the Ethereal is the realm of spirits. When you sleep, meditate, or even tap into your creative mind to play music or create art, the planar forces you tap into is the Astral. If you die, fall into a coma, or even so much as become invisible, you are tapping into the Ethereal.
Neither the Astral or Ethereal planes have been fully explored, as they are vast beyond imagination. This, the elven scholars state, is the reason why it seems there would be multiple planes. According to the Adurant there are only the two, but they are so vast as to allow different biomes to exist within them. In addition, anyone travelling into these planes shifts the plane's appearance and nature slightly and those who are able to create demiplanes can in fact mold the fabric of the planes to make nearly whatever they might imagine.
There is no evidence that Gods would exist in the other planes, but creatures matching many religious iconography of angels and demons do exist in the planes. Whether this is because the planar spirits take on the form due to preconceived notions of entering Material creatures, or because they are in fact the spirits' true forms may never be found out - it would, after all, be impossible to tell.