In the United States, Italian sausage most often refers to a style of pork sausage noted for being seasoned with fennel and/or anise as the primary seasoning. In Italy, however, there are a wide a 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 are sold as 'Italian sausages'. They are very popular in magazine recipes, used in place of regular sausages.
See the Italian Sausages category for more information.