Cassata is a sort of a Sicilian version of trifle.
It is a moulded cake, usually rounded because it is formed in a bowl, but it can also be made in a rectangular pan, or in a springform pan (though granted this is a modern innovation.)
To make Cassata, you layer the sides of the mould with thin slices of sponge cake (called "Pan di Spagna" in Italian), and set this aside while you making the filling.
A common filling is ricotta based. You press ricotta through a sieve, then blend the ricotta with a sugar syrup, stir in flavourings (vanilla, cinnamon, Maraschino liqueur), nuts, chopped or grated chocolate and candied fruit.
You press this mixture into your lined mould, level the top off, and top with more thin slices of sponge cake, and chill it.
When chilled and set, you turn it out of the mould, and spread it all over with apricot jam, and cover it with fondant (typically pale green), or whipped cream, and serve.
Filling variations vary by location and family tradition within Sicily. You can also do layers of the cheese mixture with the sponge cake inside.
Some versions of Cassata have the liqueur brushed over the cake slices instead of mixed in with the cheese, and use brandy instead of Maraschino liqueur. A version made in Erice (in Sicily) coats it in almond paste, and uses citrus preserves in the filling instead of the apricot jam layer.
Cassata is served at every festive occasion in Sicily, and at all the great Sicilian celebrations, such as weddings, Easter, and Christmas.
No one makes it anymore, though; they buy it from a pastry shop. These are usually decorated with candied fruit pieces.
The Portuguese also make a version, but use Lady's Fingers instead of sponge cake and whipped cream instead of the ricotta.
In traditional versions of Cassata, the ricotta is sheep's milk ricotta. If you can't get sheep's milk ricotta, mascarpone is a good substitute.
Being based on sponge cake ("pan di spagna"), some speculate that Cassata may have emerged during the period of Spanish rule from 1282 to 1458 AD (with Spanish viceroys appointed until the early 1700s.)
Some speculate that the name Cassata may have come from Arab name for a deep bowl, "qas'ah." Arabs did invade and occupy Sicily for several hundred years (878 AD - 1091 AD.)
Then again, the world may equally have come from the Roman name for cheese, "caseus."
Others say it just means "cased", as in the case being the bread crust it is encased in.