With all due respect to Tom Dissonance, he completely missed the point of what makes cheese kosher or unkosher, and why supervision is necessary in the process. The first thing that is essential for making kosher cheese is obtaining milk from a kosher animal. Under the laws of kashrut, any product that comes from an unkosher animal, be it the meat, the bones, the hooves, or the milk, is also considered unkosher and unfit to be eaten. This is why kosher gelatin and Jell-O must be specially prepared -- traditional techniques derived the gelatin from the hooves of livestock and it is easy to see how this is problematic when pigs, horses, or other nonkosher animals are involved. So the milk must come from a healthy, kosher animal. In the United States and Europe, this is not much of a problem because the overwhelming majority of cheeses are made from cow, goat, or sheep's milk, all of which are kosher animals. This does not mean, however, that all cheese found in these areas may be eaten. It is possible to find cheese that is made from reindeer, camel, or kangaroo milk and it is important to be aware that these are non-kosher products.

The other cause for concern in the cheese making process is the use of rennet. Rennet (follow the link for more info) is a milk-curdling substance that is used to speed up the acidifying of the milk. This causes the milk to form the soft curds which will later become the cheese itself. The problem with rennet which affects vegetarians as well as Jews, is that it is derived from rennin, an enzyme found in the stomach of cows. As Barbara Ciletti says in her book Making Great Cheese,

Five hundred years ago, the dairy farmer or cottage cheesemaker extracted the fourth stomach from a newly slaughtered, once healthy, milk-fed calf. The fourth stomach, which contained the rennin, was then carefully washed and hung to dry in the air. The farmer added dehydrated pieces of the stomach directly to curdled milk, which solidified into a mass ready for the next step in the cheese-making process. (p. 30)

While the technique is much more refined today, because the rennin is derived from a cow's stomach, it is still considered to have the quality of "meat" (fleishig)1. This is problematic because one of the laws of kashrut dictates that milk may not be mixed with meat. Since the enzyme is considered a meat product, it cannot be used at anytime in the cheesemaking process because it will conflict with the milk. Any cheese that uses rennet derived from meat (will list "enzymes" in the ingredients2) is not kosher. In order to make kosher cheese, vegetarian, non-animal based forms of rennet must be used.

As with all kosher food, the preparation process must be supervised by a Mashgiach, a person specially trained not only in the laws of kashrut but in the techniques and practices of the food service industry. It is important to make sure that the livestock being milked is healthy and in good condition, that preparation equipment is not mixed between meat and dairy products and that a general level of quality is maintained. In the situation mentioned in the above writeup, the mashgiach would have been especially important, making sure that the correct rennet was used and that all of the machines were properly cleaned and kashered because they had been used to make non-kosher cheese previously.

1In kashrut, foods are of three basic types, usually categorized by their Yiddish names: Without going into too much detail, every food falls into one of these three categories. Milk is obviously milchig, whereas the enzymes derived from the cow's stomach are considered fleishig and so cannot be mixed.

2Thanks to George Dorn's rennet writeup for what to watch out for on labels.

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