What varities of chili peppers are most commonly used in Italy
Peperoncini (=literally little big peppers)
Grown and used in all Italy

The green ones - Peperoncini verdi
Used in north Italy, made under vinagre, typical Milano's recipe.
They make it in airtight glass jars and open them in December for Christmas (or whenever)
The red ones - Peperoncini rossi
Grown and used most af all in shouth of Italy. They are put to dry, then made in powder, like chili. Can be from medium hot to high hot. Used for seasoning many recipes in all Italy. Many types of red small peppers are used to be dried and for seasoning recipes.

Peperoncini verdi fritti
http://www.cookaround.com/yabbse1/showthread.php?t=8358 (see the photo 1 and 2)
Typical of south Italy.
They are fried in oil, then dried on a cooking paper, then seasoned with salt.
"Pepperoncini" is a contaminated word by a wrong spelling pronunciation of southern Italy, and "friggitelli" (like "puparuolilli do’ sciumm", in the text) is a local/dialectal denomination, not even known in north Italy.
Peperoncini rossi corti ripieni (=red short filled pepper)
Typical of South, filled with tuna and other ingredients.

Peperoni = (literally = big peppers)
Used in all Italy (the two varieties = the shorter are sweeter) - but any color changes the flavor, so the result, depending on the blend color, changes.

They are used most commonly used to make "peperonata", made with onions, oil and tomatoes (some like a ratatouille). With celery in north of Italy. They can be from sweet to hot.

Can be made in oven also, usually filled. Or passed on any fire to burn the skin, then peeled from the burned skin, reduced in large stripes, put into a container, seasoned with garlic, oil and salt and covered with oil. They let rest at least 24 hours for the flavors to blend.