Humans have been using rennin and rennet to make cheese for thousands of years. In fact, documents written by Homer suggest the process goes as far back as 800 B.C., but there is typically some confusion between what these two components are and what exactly they do.
Rennin is an enzyme that is found in rennet, which in turn is found in the gastric juice of a mammal's stomach. The function of rennin is to curdle the milk the animals digest, thereby making it easier to absorb; for this reason it is a key element in making cheese from cow's milk. While rennet contains many different enzymes, rennin is the only one that serves to coagulate milk. For the home cheesemaker, rennin can be bought in tablet or powdered form from a supermarket; look for it in the dessert section.
Rennet is a combination of different enzymes that help mammals digest milk. Each animal produces a different type of rennet, therefore specific types are used for certain cheeses, such as kid goat rennet to make goat's cheese and lamb rennet for sheep's cheese. It is usually only extracted from the stomachs of young animals, as rennet in older animals contains little or no rennin.
Vegetarian rennet is manufactured to provide a non-animal alternative for people who do not eat meat; enzymes from plants and fungi are used. Another motive for producing non-animal rennet is because of the difficulty in sourcing sufficient animal stomachs for production. Vegetarians should always check labels to see whether cheese has been made with animal rennet or a vegetarian alternative.
Rennet is extracted from the stomach by adding acid, which separates the different enzymes. The rennet extract is filtered several times until a concentrated substance is achieved. Manufacturers make 1 kilogram of cheese that usually contains around 0.0003 grams of rennet. If you are buying rennet to make your own cheese, you simply dissolve or stir it into water before adding it to the milk.