Heaven, Hell, the Garden of Eden
Paradise Lost takes place right around what Christians would say is the beginning of human history. The poem begins after Satan's unsuccessful rebellion and the creation of the universe. Milton's conception of the cosmos is slightly strange, but basically at one end is Heaven, at the other is Hell, and in between is a place called Chaos (described in some detail in Book 2). Now, our universe – the earth, the stars, Jupiter, the moon, etc. – is enclosed in some type of spherical structure that is attached to Heaven by a chain. Just imagine a doll house (Heaven) floating in the air with a balloon attached to the bottom of it (inside the balloon is the universe as we know it).
The first two books are set in Hell. Milton spends a good amount of time describing Hell's surroundings, even adding the little detail that Hell becomes a frozen, arctic tundra once one travels some distance from where the fallen angels initially congregate. After describing the frozen part of Hell, Milton says something to the effect of "it's so cold, it's hot." Very clever.
Heaven is the setting of Books 3 and 6; Milton segues from Hell to Heaven right away in order to highlight the contrast between them. Unlike Hell, which is really hot and really cold, Heaven is temperate (i.e., not subject to extreme temperatures); Hell is dark ("darkness visible" reigns there) while Heaven is bright. Even when it is "nighttime" in Heaven, it's not really dark, only dim. Hell isn't comfortable, but Heaven is the most peaceful place imaginable.
The Garden of Eden is, for the most part, the setting of the rest of the poem. Paradise is exactly what you would expect. Every single sweet-smelling plant and tasty fruit exists there; all the animals get along (lions and tigers appear to be vegetarians because Milton tells us they don't chase other animals); and the weather is always perfect. What really drives the point home is the part where Adam and Eve drink from a cool stream. They don't have a cup, so they use a hallowed-out piece of fruit. Doesn't it remind you of Hawaii? Kind of? Just a little bit?
While Milton does everything he possibly can to make Paradise appear pure and undefiled, his descriptions of the Garden of Eden always end up reminding us that we no longer possess it, that such a place can only be accessed through the imaginative productions of poets like Milton. When Adam and Eve leave the garden at the end of Book 12, a "flaming brand" or sword blocks the Gates of Paradise, reminding them (and us) of its ultimate inaccessibility.