In the United States, Italian sausage (salsiccia[salˈsittʃa] 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, a wide variety of sausages is made, many of which are quite different from the product commonly known as 'Italian sausage' in the United States.
The most common varieties are marketed in US grocery stores as "Italian Sausage" are hot, sweet, and mild. The main difference between hot and mild is the addition of hot red pepper flakes in the spice mix of the former. The difference between mild and sweet is the addition of sweet basil in the latter.
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.