Made from coarsely ground pork mixed with seasonings including fennel seed, sweet Italian sausage is one of two types of Italian sausage (the other being hot). It is a fresh sausage (as opposed to cured or dried) and must be well cooked before eating. Most often in sold in links, it's also sometimes available as patties or bulk sausage. Though Italian sausage is traditionally made with pork, chicken and turkey versions are also widely available (and are sometimes lower in fat).
Other fresh sausage can substitute, but sweet Italian sausage has a distinct garlic-anise flavor, so if you're subbing something else, you may want to add a bit of garlic and/or fennel seeds to the recipe.
To easily remove the meat from its casing, split the casing lengthwise with a sharp knife.
For longer storage, freeze Italian sausage links (well-wrapped) for up to 3 months.