In the United States, Italian sausage (salsiccia, in Italian) most often refers to a style of porksausage noted for being seasoned with fennel and/or anise as the primary seasoning. In Italy, however, there are a wide variety of sausages, many of which are quite different from the product commonly known as 'Italian sausage' in the United States.
The two most common varieties marketed in US grocery stores as "Italian Sausage" are hot and sweet (or, depending on what region of the US, mild). The main difference between hot and sweet (mild) is the addition of hot red pepper flakes in the spice mix of the former.
In Australia, a variety of mild salsiccia fresca (literally meaning "fresh sausage") seasoned primarily with fennel is sold as "Italian sausage," which is popular in magazine recipes and often used in place of regular sausage.