What happens in Canto VI of Dante's Inferno?

Canto VI
Having just left Francesca and Paolo, the lovers guilty of elevating their own lust and desire over God, Virgil and Dante descend into the third circle. Their progress however, is guarded by a three-headed dog whom Virgil knows as “Cerebus.” This hell hound keeps the sinners in their torment and torments them himself:

Cerberus, monster cruel and uncouth,
With his three gullets like a dog is barking
Over the people that are there submerged.
Red eyes he has, and unctuous beard and black,
And belly large, and armed with claws his hands;
He rends the spirits, flays, and quarters them.
Howl the rain maketh them like unto dogs;
One side they make a shelter for the other;
Oft turn themselves the wretched reprobates.

Busy with his work, it takes a moment for Cerberus to notice the intruders. When he does, the dog-monster snaps and snarls. To silence the beast, Virgil throws mud into the dog’s mouths. Surprisingly, the dog settles down to chew on his “treat.” Like the gluttons he guards, Cerebus too is obsessed with food.

When Cerberus perceived us, the great worm!
His mouths he opened, and displayed his tusks;
Not a limb had he that was motionless.
And my Conductor, with his spans extended,
Took of the earth, and with his fists well filled,
He threw it into those rapacious gullets.
Such as that dog is, who by barking craves,
And quiet grows soon as his food he gnaws,
For to devour it he but thinks and struggles,
The like became those muzzles filth-begrimed
Of Cerberus the demon, who so thunders
Over the souls that they would fain be deaf.

Once they can get closer, Dante observes sinners who are sunken in mud. These condemned souls are constantly bombarded with filthy water and they are pounded by hail. All of the sinners are laying down. However, as Dante and Virgil pick their way across the muck, they pass one sinner who sits bolt upright. His name is Ciacco and he wants to convey some information to Dante.
Ciacco knows Dante can return to the world of light. He calls out to the poet, but Dante knows him not. Perhaps trying to be polite, Dante says that he may be unable to recall the shade’s name as he is overcome by pity and disgust:

"The anguish which thou hast
Perhaps doth draw thee out of my remembrance,
So that it seems not I have ever seen thee.
But tell me who thou art, that in so doleful
A place art put, and in such punishment,
If some are greater, none is so displeasing."

Ciacco, though in Hell, nonetheless has some important information about the fate of the warring political parties above. He warns that the factions are committing sins for which they will be punished:

"They, after long contention,
Will come to bloodshed; and the rustic party
Will drive the other out with much offence.
Then afterwards behoves it this one fall
Within three suns, and rise again the other
By force of him who now is on the coast.
High will it hold its forehead a long while,
Keeping the other under heavy burdens,
Howe'er it weeps thereat and is indignant.
The just are two, and are not understood there;
Envy and Arrogance and Avarice
Are the three sparks that have all hearts enkindled."

Ciacco appears to be done with speaking but Dante prods the former Florentine to tell him the fate of some particular comrades Dante had known in life. He prods:

"I wish thee still to teach me,
And make a gift to me of further speech.
Farinata and Tegghiaio, once so worthy,
Jacopo Rusticucci, Arrigo, and Mosca,
And others who on good deeds set their thoughts,
Say where they are, and cause that I may know them;
For great desire constraineth me to learn
If Heaven doth sweeten them, or Hell envenom."

Weary, Ciacco replies that their fates are worse than his own, and Dante will discover what punishment was given them as he continues his journey:

"They are among the blacker souls;
A different sin downweighs them to the bottom;
If thou so far descendest, thou canst see them.
But when thou art again in the sweet world,
I pray thee to the mind of others bring me;
No more I tell thee and no more I answer."

Ciacco speaks no more. Dante and Virgil move on.