Kosher sausages conform to the same manufacturing rules as other sausages, the difference lies in meat selection. The meat selection is pretty much defined by the Jewish Bible: You may eat any animal that has a split hoof and chews the cud: the cow, the sheep, the goat, the deer, the antelope.
Kosher foods fall into three categories: meat, dairy, and "pareve," sometimes spelled "parve" that means neutro.