A raviolo is traditionally composed by two square or round layers of thin egg pasta dough and a sealed filling of vegetables, meats, or cheese between them. Who invented them? We don’t know, but Giovanni Boccaccio in the Decameron (1349-1353) wrote: "the only thing they did was making and cooking maccheroni and ravioli " ("...niuna altra cosa facevano che far maccheroni e raviuoli e cuocergli..."), so they must have been already famous in the XIV century. In Rome, they were well-known in 1549 when Bartolomeo Scappi served them with boiled chicken to the papal conclave. Ravioli are traditionally made at home because they are an easy task: two and a half cups of flour of durum wheat (also called semolina, or all purpose flour if you cannot find it); 3 eggs; a pinch of salt; a tablespoon or two of tepid water, if the dough is too dense.
Once you knead the dough, which takes about 15 minutes, form a ball with it, cover it with plastic wrap and let it rest for one hour. Dust a rolling pin with flour to prevent the dough from sticking and roll it out until it is thin enough, ready to be cut.
The filling? Go crazy! The traditional ones are a mix of cheeses; ricotta & spinach; already cooked ground meat (beef or chicken or sausage) mixed with egg and parmigiano; already cooked minced vegetables also mixed with egg and parmigiano; and for quite some time chefs have been using seafood.
In Rome, Latium, Abruzzo and Marche, the filling is made with ricotta, spinach, nutmeg, and black pepper; in Sardinia with ricotta and grated lemon rind; in Emilia Romagna, Tuscany, and Piedmont with ground beef or pork; in Liguria with vegetables, herbs, and cheese; every region and area has its own spin.
You can have them with broth, dressed with butter, sage and parmigiano, or with your favorite tomato sauce. And you cannot get tired of them, ever.
A recipe from Accademia Barilla: http://www.academiabarilla.com/italian-recipes/basilicata/ravioli-monticchio-style.aspx