Calvino wanted to be Italy's Brothers Grimm. Not in the sense of collecting the tales, which folklorists had been doing, but creating a popular collection. This is the result, down to the 200 tales. He notes that he also, like the Grimms, spruced up the tales. Unlike them, he put in end notes about what tales he combined, and what little details he added. (He went by the language and not by the country's borders.)
It's a wide variety, including some religious legends and animal fables and tales about fools, but it's mostly fairy tales. A few are retellings of Greek myths, like Perseus, or the Cyclopes, after they are run through the folkloric lathe to pare them down to simplicity. A few others are obviously derived from the chivalric romance, also pared down.
A familiarity with many tales will often recognize them and what fillips Italy added -- which sometimes Calvino noted as well. "A Man Wreathed With Seaweed" is a common European tale, except that elsewhere the hero has to be lowered through a well or other hole in the ground "The Ship With Three Decks" combines an opening like "The Lord of Lorn" with a tale type like "The Firebird, the Princess Vasilisa, and the Horse of Power." "Bella Venezia" gives a Snow White tale with a innkeeper not a queen, and bandits instead of dwarves -- and guess how the bandits get into their cave. A girl must be silent for seven years to save her brothers not from being birds, but from Hell -- and instead of her husband the king only be persuaded to execute her at the very end of that time, she has to flee and ends up with bandits. "Misfortune" is one I've only run across in Mediterranean tales -- a heroine whose Fortune is bad. We have children born as apples and rosemary plants, crabs and snakes. In one, two royal children reclaim their kingdom from their wicked uncle before the brother foolishly marries a beggar woman, whose new position goes to her head and makes her hate her sister-in-law. A Cinderella tale where first the heroine sneaked into the prince's castle to do mischief.
And a lot more.