Cheese is usually made out of milk and cultures. In some cheeses, they add in animal rennet which is usually the stomach lining of baby cows. Some of the brands which include rennet are Kraft, and even some organic cheeses. In general, cheese is considered one of the vegetarian protein sources & due to this rennet addition its now deemed not eatable by vegetarians. The way the rennet is obtained is pretty cruel & this should be stopped or at least advertised in the label so vegetarians can stop consuming them. 
For more info/ research: https://www.vrg.org/journal/vj2008issue3/update_renet.htm