Many Jews follow the laws of kashrut; the dietary laws set down in the Torah (the first five books of the Bible). The following is an outline of these rules and is by no means definitive. For more detail, consult the Old Testament.
The first step in observing the laws of kashrut is to determine whether the ingredients in the food to be eaten are in fact kosher (the Jewish laws regarding food preparation). The only foods limited by this are those derived from animals. These fall under four categories: land animals, birds, insects, and aquatic animals. Land animals, to be kosher, must have cloven hooves and chew their cud. Thus, cows and giraffes are kosher, but pigs and tigers are not. To fit the requirements, birds must not be birds of prey or scavengers; chickens are fine, vultures are not. The only kosher insects are grasshoppers and locusts. Aquatic animals must have fins and scales. This may not seem to have any effect, but it rules out all shellfish, whales, dolphins, seals, and certain types of shark and other fish with rough skin instead of scales. Vegetables, milk, and eggs are automatically kosher.
After limiting the types of animals that may be eaten, the laws of kashrut dictate how they must be killed and prepared. There are no rules about killing or preparing insects and fish, but the slaughter of birds and land animals must follow a strict procedure. They must be killed with one stroke of a sharp, nickless knife across the throat, which is supposedly painless. This cut is performed by a Jewish butcher, or shohet, under the supervision of rabbis. After killing the animal, the blood must be drained from the meat, which is usually done by soaking and salting. The body must also be inspected closely for any deformities or illnesses, which render it non-kosher. Animals which have died a natural death may not be eaten.
The third set of rules determine what foods may be eaten together. The rabbis divide all foods into three categories: milk, meat, and pareve. Pareve foods include vegetables, fruits, fish, and eggs, and may be eaten with any other foods. The meat category includes any poultry or land animals. All dairy products, such as cheese and yogurt, are included under milk. According to the prohibition 'You shall not boil a kid in its mother's milk', milk and meat may not be eaten together. To prevent any possible mixing of the two, separate dishes for preparing and serving them are necessary. If a dish is used for both, it is non-kosher and must be koshered, usually by immersion in boiling water. Before then, anything prepared or served in it is non-kosher. There is also a waiting period after eating meat before milk may be eaten. This varies according to different traditions, but is generally three to six hours. There is no waiting period after eating milk products, as long as the mouth is rinsed out between the two with a pareve drink.
There is a theory that kashrut was actually designed to keep the Israelites healthy. There is some merit in this idea; the inspection of animals, the exclusion of fatty animals such as pigs, and other laws are often suggested today by health experts. However, some of the rules seem pointless, such as the separate dishes. It seems likely that these laws were included to keep Jewish people united. Jews around the world follow the exact same dietary restrictions, and that is one of the factors that has kept the religion strong throughout its history.