In "Book I" of Paradise Lost Milton gives us some very memorable images of hell; many of them allude to the Bible, and others from his own mind. Here are some of them:

"bottomless perdition" (I. 47)

"fiery gulf" (I. 52)

"A dungeon horrible" (I. 61)

"one great furnace flamed, yet from those flames / No light (I. 62-63) My favorite.

"Regions of sorrow, doleful shades, where peace / And rest can never dwell, hope never comes / That come to all" (I. 65-67)

These are incredible images of Milton's hell. The first one alone is astonishing. In the two words "bottomless perdition" Milton gives us a sense of the magnitude of the punishment; it is eternal; this idea is accomplished via the word "bottomless." How does one escape a bottomless anything let alone a "perdition?"

The other that is quite mind-boggling is "flames" that cast "No light." This paradox certainly further enhances the "bottomless perdition" idea, doesn't it? The image reveals just how hopeless it is in hell, for light represents hope; Milton confirms this lost of hope when he says Hell is a place

where peace

And rest can never dwell, hope never comes

That comes to all (I. 66-67)

I'm afraid that a complete description would take quite a while (more space than is available here), because it would involve things like describing the mindset of Satan and the fallen angels.

However, you can get a good discussion of some elements of hell in the summary and analysis of Book 1. There and in the poem itself you'll find it mentioned that there is a lake of fire in hell, that it is "horrid," "dismal," a "dungeon," full of pain and despair, and so on. It is usually silent, until Satan speaks, but a "horrid" silence. It is full of devils who when they do act, make terrible noises and do unspeakable things.