Rennet is a set of enzymes that coagulates milk, separating it into curds and whey. It's an aid to milk digestion, and it's found in every mammalian stomach.
It's also an essential part of the cheese-making process. Traditionally, cheese was made with rennet extracted from calves' stomachs, but this method is rare now and most cheese producers use non-animal substitutes made from plants or fungi as coagulants. Rennet made a big splash in the news this week... Mars' parent company Masterfoods had British vegetarians in an uproar this week when it announced a switch to rennet in their chocolate products. The backlash was so intense that yesterday they announced a change of plans and said they plan to keep their chocolate free and clear of animal products.