The word kosher means fit ie. fit for use. The laws governing which things are kosher and which are not are known as the laws of Kashrut. The opposite of kosher is traifeh.

Most people know that Jews must not eat pork, but Kashrut is much more extensive than that. It covers not only the foods that are permitted but the way they must be prepared, the way they may be mixed, the vessels that they are cooked in, the crockery that they are eaten from and the cutlery they are eaten with. The following is a summary of the basic dietary laws:

• Some animals may not be eaten at all (this includes the flesh, organs, eggs and milk).
• Animals and birds that may be eaten must be killed in accordance with Jewish law.
• All blood must be removed from the meat before it is eaten.
• Some parts of even permitted animals may not be eaten.
• Meat (including birds) and dairy products may not be eaten together (Jews generally refer to "meat" and "milk".  Fish, eggs, fruits, vegetables and grains can be eaten with either meat or milk.
• Utensils that have come into contact with meat may not be used with dairy, and vice versa.
• Utensils that have come into contact with non-kosher food may not be used with kosher food.
• Grape products made by non-Jews may not be eaten.

The animals that are  kosher are those land animals which both have cloven hoofs and chew the cud. This includes cattle, sheep, deer but excludes pigs (don't chew the cud) as well as many other animals. Birds of prey are forbidden but other birds are generally OK. Reptiles and amphibians are not kosher. The rules of kashrut forbid consuming blood, so when an animal is killed its carcass must be drained of blood; meat must also be salted then soaked before cooking, to remove any remaining blood (rare steaks are not a feature of Jewish cuisine!). For meat to be kosher the animal must have been slaughtered according to Jewish ritual. This has become somewhat controversial of late but the ritual is based on Jewish ideas of the sanctity of life and attempts to make the animal's death as quick and pain-free as possible.

For fish to be kosher they must have both fins and scales, which excludes all shellfish and other "seafood" as well as eels, but includes most other fish.

Torah specifies that only certain insects may be eaten (eg. locusts) but since the exact identification of those species that are permitted is unclear, rabbis have ruled that all insects should be considered traifeh.

The laws of kashrut do not apply only to food and its preparation. The idea of fitness for use is more widely applied. For example, there is a biblical prohibition on making clothing from mixed fibres, thus clothes made from a mixture of wool and cotton would be traifeh.

During the festival of Pesach (Passover) there is an extra layer of more demanding kashrut laws.