UPPER HELL
Vestibule-Circle Five
Inside the Gate lies the Vestibule, where the Futile and the angels who chose no sides run. Their lives were neither hopeful nor hopeless, merely apathetic, so they are neither in Hell or out of it. Beyond Hell's "lobby" lies the river Acheron. Charon is the ferryman who takes souls across Acheron into Hell Proper.
First is Limbo, where children who died unbaptized and the Virtuous Pagans (good people who lived before the coming of Christ) reside. Their only punishment is being excluded from the presence of God, and Limbo is filled with eternal sighing. This is Virgil's home, along with a number of philosophers, poets, etc.
Beyond Limbo are the CIRCLES OF INCONTINENCE (the SINS OF THE LEOPARD). These four circles terminate in the banks of the river Styx, which surrounds the City of Dis (the gates of which separate Upper and Lower Hell).
NETHER HELL
Circle Six-Circle Nine
Within the City of Dis are demons (who try to prevent the Poets from entering--a Heavenly messenger chastises them and opens the gate) and mosques. The sinners there are Heretics, who (unlike the Virtuous Pagans) intentionally chose a path other than that of Jesus Christ. They are the first of a series of shades who committed the SINS OF THE LION. These sins are more serious and carry harsher punishments.
Beneath the City of Dis lies the Seventh Circle, wherein the Violent reside. The uppermost ring of this circle is the river Phlegethon, which runs red with boiling blood. The river trails through the other rings and over the Great Barrier. This is an insurmountably tall cliff (for Dante and Virgil) that leads to the Malbowges.
The Malbowges (or "evil pouches") are narrow trenches of rock. Spines of rock form bridges across them leading down into the Well (which will be covered momentarily). The Malbowges are the first half the SINS OF FRAUD AND MALICE, otherwise known as the SINS OF THE WOLF. This half is the "Simple" fraud and malice, less serious than the latter "Complex" section that comes after the Well in the frozen lake Cocytus.The bridges span every bowge except the sixth, where the Great Earthquake shook the stone into rubble.
After the tenth bowge stand the Giants, visible from the waist up over the rim. They mark the entrance to the Ninth Circle, which is divided into four regions. The sinners here are the Traitors. The regions have names, though it is far easier to remember them by the sinners within: Caina, Antenora, Tolomea, and Judecca.
At the very center of the lake is Satan (Dis), who stands imbedded in ice. The beating of his wings continues to cool Nether Hell. He holds the shades of the ultimate traitors (in Dante's view, at least): Judas, Brutus, and Cassius. These three unfortunates he devours eternally.
Beyond Satan is the river Lethe, which flows from the core of the Earth upwards to Mount Purgatory (perched on the antipodes of Hell's Gate). From here the Poets' journey becomes far safer, although not entirely easier.