Italian sausage recipes in the New World (America) generically produce a pork sausage that is seasoned with the distinct flavors of fennel and/or anise.
In Italy itself, where there are lots of regional sausages, the name is really meaningless.
The Italian sausage that most of us know comes in two varieties:
Sweet, which in its most basic form is seasoned with fennel, salt, and black pepper, and
Hot, which adds red pepper to the mix.
Most of us recognize the flavor of Sweet Italian sausage from sausage pizza, but it is great in any number of dishes.
It is usually made as a fresh sausage (no cure), and you can put it up in bulk or stuff it into casings.