Divine plane: This is where all the benevolent gods exist. All of them have basically limitless power, yet choose to rarely interfere with the lower planes. The true nature of the plane can't be comprehended by the human mind, so should a human have the fortune of actually getting up there, all he/she would see is that bright yellow light punctuated by even brighter spheres of pure light. The only possible side effect of a human getting up here would be blindness caused by the light, though the gods would probably send him/her back to the middle plane before that happens. They are the good gods, after all.
Astral plane: Demigods and beings of light reside here. There is neither ground nor gravity, so everything floats around by their own power. Although the denizens of this plane can be quite deadly, they aren't aggressive unless excessively provoked (or if you're a demon). Beings native to this plane typically have amazing powers of light, strong magic prowess and immaterial bodies, but not all that much physical oomph. A human caught in this plane would float around with no control, but otherwise be unharmed.
High plane: This is what we know as heaven. Angels and their kin fly around here, living a life free of cares and malice. Gravity is present (though to a lesser degree than on Earth), so anything without wings will have to walk on the clouds. Said clouds have varying consistencies; solid if you walk on them, but soft as pillows if you fall on them, and if you want to, they can spring you right back up in the air. In other words, a totally awesome place to be, and a few lucky humans even get to go here after death, if they've been kind enough. The only possible threat to native beings on this plane is a demonic invasion, but these are few and far between, and thus generally not something to worry about. Even if it does happen, angels have the privilege of being reborn by the gods, so even death isn't something to fear. You can have everything your heart desires up here, since you've earned your happiness!
Middle plane: AKA the multiverse, an virtually infinite series of universes like our own, though only very few are actually similar to each other. History has played out differently in each universe, and the length of history also varies. Some of them are barely out of creation, and some of them have already undergone total heat death. Mostly though, everything's as we know it to be, except that "neutral" beings who die just have their essence transferred to another universe, so as to live another life (with no memory of the former lives). Particularly good or evil beings might find themselves transported to the High or Lower plane instead, though.
Lower plane: Hell as we know it. Demonic beings roam here, magma lines the dark and rocky ground, and the ceiling blocks out all external light. In other words, were it not for the magma and demons throwing fireballs at each other, the plane would be pitch black. Instead, however, it is permanently bathed in a searing red light, which is additionally emphasized by the burning ambient temperature. Anything caught in here without adequate heat protection will quickly burn to death, and for outsiders, that means eternal imprisonment here. Your fate would, in this case, be frighteningly similar to the horrific tales from the church.
Abyssal plane: The plane of darkness and insanity. A dim purple light permeates the place, seemingly without any source, and the intensity of this light seems to vary chaotically, almost waving around in erratic patterns. Everything native to this plane is covered in darkness, giving it a pitch black appearance, but certain beings are able to shed this cover at will, revealing a visage of extreme supernatural horror. Any mortal being subjected to this would have a 50/50 chance of going insane or just dropping dead from the sheer horror. The pitch black ground is also littered with slithering tendrils of darkness, all of which would have no qualms about ripping the soul out of an intruders body. The native beings are obviously safe from this, and even worse, they're all extremely dangerous. The weaker and mindless beings could probably be defeated by a great enough hero, but anything with sentience wields power of similar magnitude to that of the astral demigods. Only this time, they use darkness, and are incredibly deadly in physical combat as well. These are known as abyssal demons, and should be avoided AT ALL COSTS. Normal death ain't got nothing on the horrors that await the ones defeated by abyssal demons.
Hadean plane: This is it, the darkest of the dark, the evilest of the evil, the home of the dark gods. They are every bit as powerful as the benevolent gods, and although they don't meddle with mortals either, they do it for a different reason; despite their hatred for good and desire to destroy everything, they have formed a pact with the good gods. A war between the two sides would end up obliterating everything that exists, so the dark gods did see some reason, for once. Any mortal trespassing into this plane will be subject to the whim of the dark gods, which generally involves untold horrors, intense and irreversible insanity prolonged for an eternity. Over and over. So stay away. :U