Non-kosher foods are called "treif" or "treifa." "Treif" means "torn". This is because the Torah says not to eat an animal that has been killed or torn by another animal. Jews who follow the religious rules called "kashrut" only eat some types of food that are prepared by special rules. Food that a Jew can eat is called ‘kosher’ food. The term kosher, (in Hebrew –‘kasher’) means "ritually correct."

The flesh of birds and mammals cannot be eaten with dairy. Fish, eggs, fruits, vegetables and grains can be eaten with either meat or dairy. (According to some views, fish may not be eaten with meat).

Of the "beasts of the earth" (basically mammals with the exception of rodents), you may eat any animal that has split hooves and chews its cud (food chewed twice: partly digested food that returns to the mouth, after it has passed into the first stomach). Any land mammal that does not have both of these qualities is forbidden. Sheep, cattle, goats and deer are kosher. Pig isn’t.

In regard to meat, all blood must be drained before the meat is cooked and eaten, because blood, which gives life, is sacred to God. In temple services, blood was offered on the altar separately from the rest of the sacrificed animal.

Anything in the water that has fins and scales is acceptable. This therefore excludes shellfish such as lobsters, oysters, shrimp, clams and crabs, all of which are forbidden.

For birds, the criteria are less clear. The "Torah" lists forbidden birds without specifying why they are forbidden. All of the birds on the list are birds of prey or scavengers, thus the rabbis inferred that this was the basis for the distinction. Other birds are permitted, such as chicken, geese, ducks and turkeys.

Rodents, reptiles, amphibians, and insects are all forbidden.

As mentioned above, any product derived from these forbidden animals, such as their milk, eggs, fat, or organs, also cannot be eaten. Rennet, an enzyme used to harden cheese, is often obtained from non-kosher animals, thus kosher hard cheese can be difficult to find.

For religious Jews, meat and dairy products may not be mixed or eaten together at the same meal. This also means that a household that keeps kosher must maintain separate sets of cooking implements, pans, dishes, and utensils one for meat and one for dairy products. Some households even have separate sinks. These practices derive from a rule of uncertain origin that forbids the cooking of a baby goat or lamb in its mother’s milk. It is possible the practice was forbidden for being cruel: some foetal animals, cut from the womb before birth, were considered tender delicacies. The practice of cooking a kid in its mother’s milk may also have been associated with non-Hebrew religious practice and therefore forbidden.