A frittella can be just about anything, sweet or savoury, spherical or flat.
Perhaps best known frittelle (or fritole, as the Venetians like to say it) are the sweet yeast doughnuts fried and served hot in Venice during Carnival. The classic fritelle Veneziane have raisins and pine nuts mixed in the dough, and are sprinkled with powdered sugar; others are filled with cream, chocolate or zabaglione.
Also see Tuscan bomboloni.
Another sweet form is frittelle dolci di riso, a winter favourite made with rice cooked in milk and flavoured with cinnamon, lemon and rum.
Savoury frittelle can have anything in them from salt cod to zucchini blossoms. In Liguria they make frittelle di lattuga (friscieu) out of strips of lettuce.
As in other countries, they're often associated with carnival—a treat just before Lent.
In Sicily, fritella is something quite different: a simple soup of fresh fava beans, artichokes and peas, similar to the vignarola of Lazio and Tuscany.