Did you know that sausage refers to any ground meat? Yep- not just pork- any type of ground meat. Chicken, pork, beef or even seafood. It generally has added fats, salt (for preserving) and other seasonings. It can come in bulk or cased versions and either raw or cooked.
That means that technically, ground beef is also sausage!
Sausages that come in links are called cased because they need something to help them retain their shape. Casings are edible, but not all are pleasant. Some can be tough or chewy and are best removed. They can be synthetic or all natural.
A good middle ground are natural hog casings. They need to be soaked in water first, then filled.