To be strict, the "Biscotti di Prato" are the most famous "type" of "cantucci", for which there are several slightly different recipes.
Of course they are typical of Tuscany and you won't find them everywhere in Italy, in Lazio there are the similar "tozzetti".
... oh, and usually they are eaten only after the meal, dipped in either Vin Santo or Aleatico or similar sweet wines.