A portable hole is a circle of magical cloth spun from the webs of a Phase spider interwoven with Astral Ectoplasm.* It's six feet in diameter, but can be folded as small as a handkerchief. When spread out on any surface, it creates an extra-dimensional hole ten feet deep. The hole can be picked up, folded again, and transported elsewhere. The only air in the hole is that which enters when the hole is open, so it's not a safe way to transport creatures who require air to live.
If a Bag of holding or Dimensional mine is placed within a portable hole, a rift to the
Astral Plane is torn in that place. Both the bag and the cloth are
sucked into the void and lost unless someone can retrieve it. If a portable hole is placed
within a bag of holding, it opens a gate to the Astral Plane. The hole,
the bag, and any creatures within a 10-foot radius are drawn there, the
portable hole and bag of holding being destroyed in the process.
Sources: 2nd edition Dungeon Master's Guide, 3rd edition DMG, d20 SRD, A Guide to the Ethereal Plane, A Guide to the Astral Plane, Expanded Psionics Handbook
* The actual wording in the 2e DMG is "...spun from the webs of a phase spider interwoven with strands of ether and beams of Astral plane luminaries."
The d20 SRD says "...spun from the webs of a phase spider interwoven with strands of ether and beams of starlight."
The problem with this is that the webs of a phase spider are strands of ether (A Guide to the Ethereal Plane, pages 70 and 92), so the first two ingredients are redundant. In the last part, the Astral Plane does have luminaries (A Guide to the Astral Plane, page 7), but I'm making the supposition that the light from them, if captured, is essentially the same as the ectoplasm described in the Expanded Psionics Handbook, because that seems to be the "substance" of the Astral Plane, insofar as it has any substance at all.
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