Monterey Jack cheese, the base for all the other Jack cheeses, is a semi-firm, creamy, mild-flavored cheese made from cow's milk that's aged for a short period. The derivatives, which include pepper Jack, Colby Jack, and Cheddar Jack are a mixture of Monterey Jack, other cheeses, and flavorings. Jack cheeses have fairly mild flavors and superb meltability, making them great for recipes like this.