Italian sausage most often refers to a style of pork sausage. The sausage is often noted for being seasoned with fennel as the primary seasoning.
The most common varieties marketed as "Italian sausage" are hot, sweet, and mild. The main difference between hot and mild is the addition of hot red pepper flakes to the spice mix of the former. The difference between mild and sweet is the addition of sweet basil in the latter.