The Abyssal realms are myriad pocket layers within the Abyss. Within each rift in the fabric of the planes is the Abyss, which holds numberless pocket dimensions in each rift and chasm. These Abyssal realms have their own gravity, ecology, and denizens, which are obliterated or changed at seemingly no provocation. There is no direction, no meaning, and no safe place in the Abyss - a being wandering in from another plane could easily be totally vanished when a rift closes.
Not all realms are so chaotic, however. The home realms of the demon lords seem to be relatively immune to the constant destruction and creation, though they change slightly in every "day" (for the Material Plane, as each realm can have its own sun or none at all). These places may just be more static than the others, or the demon lords may themselves keep the realm from being overly changed.
There are many realms which are just as solid as those of the demon lords, which often serve as battlefields for demonic armies, whose masters fight against each other to gain more power. Even those that have not already been purposed for combat contain the demonic races, who could have been birthed from the plane itself or migrated there when it formed.
The only constant in this always-changing plane is the River Styx, which threads its way through or alongside all of the major layers and whose tributaries are never far from any realm. Though linear movement is nigh-impossible, with the improbable loops and extra-dimensional angles, finding the Styx is the best way to make it out of the rifts.
These are the more important realms that an Abyssal traveler might know of.