The Ethereal planes are misty realms of proto-matter surrounding each Prime Material Plane. In the Ethereal planes, Nothing is solid but there may be small pockets or islands of matter known as demiplanes which are sometimes the creations of extremely powerful wizards, technologists, or demi-gods.
The cocoon of the Inner Planes, the Ethereal Plane is, in Marinj the Poet's words, "The grand and misty shore." For a body that's seen it, the description makes sense, because the Ethereal's like a great fog-bound realm with mists of green, red, silver, blue, and whatnot in between. In this a cutter can sometimes see shapes: windows to other planes or drifting globs of protomatter, the stuff of future demiplanes.
The Ethereal's more like an ocean than Marinj knew, as it has its shores and its deeps. When a berk first crosses the boundary into this plane, he's in the shallow end, called the Border Ethereal. While traveling in this region, a basher's not really here or there - he's not in the Ethereal or on his starting plane; in the Border, he's in both. He can see into adjacent planes, but he's invisible to most folks there. The boundary between the Border Ethereal and the Deep Ethereal is easy to spot. It's a big, shimmery wall of color, like the "northern lights" some primes talk about. The Deep Ethereal's like the ocean, vast and bottomless.
The ethereal mist isn't completely empty, though. Shapes rise and fall out of this void like strangers in the fog. Pure fact is, there's plenty living here, things friendly and dangerous both. There's creatures that live on the Ethereal but hunt on the Prime Material. There's others that prey on ethereal travelers, waiting to snare them in the mist...
Then there's the demiplanes. They're islands of matter out in the deeps of the Ethereal. Demiplanes are like regular planes, only they're smaller and have definite borders. Sages figure that, in some millennia hence, these demiplanes might become full-fledged planes themselves, but this is only theory since it's never happened (that the sages know about, anyway). No basher knows how many demiplanes exist. Wizards keep making new ones all the time, it seems. Some of them are well mapped, but others are barely known.
Note: The Ethereal Plane appears in more detail in the Guide to the Ethereal Plane from TSR. Its use here is for the purposes of providing context for the campaign only.