Just another name for ravioli, really. Mantua is known for squash or pumpkin-filled tortelli, tortelli di zucca. Tortelli d'erba, or tortelli verdi from Emilia-Romagna are filled with spinach or chard, and cheese. In the Marche they make tortello al rosso d’uovo, stuffed with ricotta, spinach, and egg yolk, in a light tomato and basil sauce.
They take them seriously in Crema, as a dish for holidays and special events. There is an academy dedicated to the local tortelli cremaschi, stuffed with amaretto cookies, raisins, cheese and lemon. Similar tortelli with sweet fillings (some fried) are made in many regions during carnival.
Just to make things confusing, sometimes you will see tortellini or tortelloni called 'tortelli' (or even tordelli.
Little Sagginale, near Borgo S. Lorenzo north of Florence, honours its favourite pasta with a Sagra del Tortello in June and August.