Rennet ( /ˈrɛnɨt/) is a complex of enzymes produced in any mammalian stomach, and is often used in the production of cheese. Rennet contains many enzymes, including a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). They are also very important in the stomach of young mammals as they digest their mothers' milk. The active enzyme in rennet is called chymosin or rennin (EC 3.4.23.4) but there are also other important enzymes in it, e.g., pepsin and lipase. There are non-animal sources for rennet that are suitable for consumption by vegetarians.
Read more about Rennet: Production of Natural Calf Rennet, Alternative Sources of Rennet