Kosher rennet, sold in Halal markets as shangesh, is key to the making of a Kosher cheese. Here's what the web site Jewish Recipes writes about it:

Rennet is the enzyme used to turn milk into curds and whey; most forms of rennet derive from the lining of the stomach of an animal. Kosher rennet may be made from the stomachs of kosher animals slaughtered in conformance with the laws of kashrut, or may be made from vegetable or microbial sources. The Mishna and Talmud (in Avodah Zarah and Hullin) state that cheese made with rennet derived from a non-kosher animal is non-kosher. Orthodox authorities follow this ruling, and hold that rennet is a "d'var ha'ma'amid" (דבר המעמיד), something that changes the status of the food so much that any amount makes the food it is added to non-kosher. Conservative authorities classify rennet as something that has changed so much from its original form that it is a "d'var chadash" (דבר חדש), "something new", and thus is no longer un-kosher. In practice Orthodox and some Conservative Jews eat only cheese made with kosher rennet, while other Conservative Jews follow the Conservative ruling and eat any hard cheese.