The following is via OU.org:

How is cheese made kosher?

As with any food, all of the ingredients in the cheese as well as the equipment used during the manufacturing process must be kosher. However, a special prohibition makes kosher certification of cheese a bit more challenging: the ban on gevinat Akum, which means that cheese not made under special rabbinical supervision is not kosher.

What is the source for gevinat Akum?

The Talmud (Avodah Zarah 29b, 35a-35b) states that the sages of the Mishnaic period forbade eating cheese manufactured by non-Jews. Although the Talmud offers various reasons for this prohibition, most halachic authorities maintain that the ban was made because of the use of rennet in cheese making. Since rennet was traditionally derived from the lining of a calf ’s stomach, Chazal forbade cheeses not made under onsite rabbinical supervison because of the likelihood that they contained rennet from calves that had not been slaughtered in accordance with halachah.

It is important to note that the prohibition against gevinat Akum is not at all related to the kosher regulations regarding milk (chalav stam and chalav Yisrael—unsupervised milk and milk under onsite rabbinical supervision). Those who consume chalav stam are fully bound to adhere to the prohibition against eating gevinat Akum. Gevinat Akum is deemed non-kosher under all conditions, rendering the utensils and cookware used in making and serving it non-kosher as well.

Can the miniscule amounts of rennet used in hard cheese render the product non-kosher?

A product containing a minuscule amount of a non-kosher ingredient is often regarded as kosher, as the non-kosher substance is batel, or nullified. However, rennet used in hard cheese cannot be batel because of the halachic axiom that a non-kosher ingredient that gives a product its form—called a davar hama’amid—is never nullified (Yoreh Deah 87:11). Even trace amounts of such an ingredient can affect the kosher status of a product. Rennet is one of the most potent food enzymes, and it is therefore used in hard cheese in minute amounts; nevertheless, it cannot be batel.

Aren’t some cheeses made from non-animal derived rennet?

In today’s world of advanced food technology, much of the rennet used is microbial, that is, artificial. Nevertheless, mainstream halachic literature posits that Chazal banned all cheese made without onsite rabbinical supervison, irrespective of the presence of animal rennet, as a precaution against the consumption of actual non-kosher animal rennetbased cheese (Rambam, Hilchot Ma’achalot Asurot 3:14 and Shulchan Aruch ibid., 115:2). Thus, cheese made from artificial rennet (as well as Portuguese hard cheese made from thistleflower rennet) is not kosher when manufactured manufactured without onsite rabbinical supervison. It should be noted that the bulk of today’s cheese manufactured in mainland Europe does contain animal rennet. Furthermore, lipase—an enzyme added to some cheeses to hasten the breakdown of fat and endow a more powerful flavor—is almost always animal-derived (lipase is extracted from the tongues of domesticated animals), although artificial lipase substitutes are becoming more widespread. Romano cheese is usually treated with goat, lamb or kid lipase, and blue cheese often contains calf lipase.

How does one make gevinat Yisrael?

Some halachic authorities rule that to satisfy the gevinat Yisrael requirement, a rabbinical supervisor must be present to supervise the cheese making and ensure that only kosher rennet is used; others hold that a rabbinical supervisor must personally add the rennet (similar to “bishul Yisrael ” and “pat Yisrael,” which are satisfied only if the rabbinical supervisor is actually involved with cooking or baking the food). The OU follows both halachic opinions and insists that rabbinic field representatives supervise all kosher cheese productions and add the rennet as well. In modern cheese facilities, rennet is often not added manually. Rather, it is dosed into cheese vats via automated rennet feeders. In such cases, the rabbinic field representatives activate the rennet feeders for each vat of cheese produced. Cheese made in Jewish-owned plants is automatically considered gevinat Yisrael, thereby alleviating the need for full-time rabbinic supervision or involvement (Shach on Yoreh Deah 115, s.k. 20).

Does gevinat Yisrael also apply to soft cheeses?

This, too, is a point of dispute. Some halachic authorities maintain that gevinat Yisrael applies to all cheeses. Others contend that only cheeses with rennet are subject to this rule. The OU and most of the other kosher certifying agencies adopt the latter position, and on-site full-time supervision is thus not required for acid-set cheeses. (Of course, the ingredients and equipment must be kosher nonetheless, and a reliable kosher symbol must be present on the package.)

Why is kosher hard cheese so expensive?

The cost of sending rabbinic field representatives to far-flung places to supervise hard-cheese production for days on end is significant. Kosher cheese manufacturers will naturally need to charge more for their products to cover the costs involved.

Furthermore, nearly all domestic and European hard-cheese plants are non-kosher when not doing special kosher cheese productions. These plants schedule kosher campaigns sporadically in the midst of their normal nonkosher activity. Thus, aside from supervising the cheese manufacturing process, the rabbinic field representatives often need to kasher (or supervise the kashering of ) each plant before every kosher production. This can take days to complete, and it is not simple work.

Must one wait six hours to eat meat (for those who wait six hours after meat to eat dairy) after eating aged cheese?

One must wait six hours to eat meat after eating cheese that is aged for six months or longer. The following are a few of the more popular aged cheeses that are aged for six months: Dry Monterey Jack, Cheddar (Medium, Sharp and Aged), Marble Cheese, Parmesan, and Picante Provolone.

For the complete list, please see OUKosher.org’s Aged Cheese List.

Do children have to wait between meat and dairy meals?

Although children who do not yet have a basic understanding of a given halachic principle are not bound to observe it, it is prohibited for an adult to directly cause a child to violatehalachah. Therefore, one is not allowed to feed a child—or even an infant—meat and dairy together. (The general rule is that an adult may not make a child transgress a Biblical prohibition. Some halachic authorities make exceptions for rabbinic prohibitions in certain cases. The overall topic is very complex and is beyond the scope of this article.) Very young children who do not understand the basic principle of not mixing meat and dairy do not need to wait. Once a child has a minimal understanding of the prohibition, he should wait an hour after eating meat before eating dairy. As a child grows older, he should be encouraged to wait longer. The exact amount of time to wait depends on the child’s maturity and ability to wait; other factors may also be considerations. (For example, if a child’s younger siblings are allowed to wait less time, and this may cause him to view the halachah negatively, this must be factored into the decision.) Consult a competent rabbinic authority for guidance.

(Source: OU.Org)