Kosher is the Anglicized form of the Hebrew term kasher, which literally means "good" or "proper," but came to indicate an item "fit for ritual use." Kashrut thus means "fitness" for ritual use. The Hebrew word for non-kosher is trayf, derived from the word terayfa, "torn", stemming from the commandment not to eat meat that has been "torn" by other animals. Any style of food may be kosher or non-kosher, from Chinese to Mexican to Jewish food. Similarly, traditional Ashkenazic Jewish foods like knishes, bagels, blintzes, and matzah ball soup can all be non-kosher if not prepared in accordance with Jewish law.
While Jewish Dietary Laws originated in the Bible (Leviticus 11 and Deuteronomy 17), they have been codified and interpreted over the centuries by rabbinical authorities, and the definitions of kosher have evolved in response to changes in the food industry, the Jewish people, and world culture.
Rules of Kashrut
The following is a rough outline of the rules of kashrut.
- A land animal may be eaten if it chews its cud and has split hooves.
- Animals which live in the water must have fins and scales.
- Blood is forbidden even from kosher animals.
- The sciatic nerve and its adjoining blood vessels may not be eaten.
- Insects are forbidden.
- Rodents, reptiles, and amphibians, are all forbidden. 
- Vegetables and fruits must be checked for insects.
- Milk and meat and or poultry derived foods must not be mixed.
- Animals must be killed in a specific way by a shohet (the exception being fish).
- Separate silverware must be used for fleshig (meat and poultry) and milchig (milk) foods.
- Utensils (pots, pans, plates, flatware, etc., etc.) must also be kosher. A utensil picks up the kosher "status" (meat, dairy, pareve, or treif) of the food that is cooked in it or eaten off of it, and transmits that status back to the next food that is cooked in it or eaten off of it.
- Most Jews who keep Kosher wait 3 to 6 hours after eating fleshig to eat milchig though the reverse is not true. Others wait other amounts of time.
- Fish, eggs, fruits, vegetables and grains can be eaten with either meat/poultry or dairy.
- Amongst poultry, birds of prey are considered treyf.
- Wine is not kosher if it has been touched by a non-Jews unless the wine has previously been boiled. This is due to the concern that the wine might be used for idolatrous purposes. 
- When, after the ritual slaughtering, an animal, apparently sound during its life, is found to have been diseased, its milk, or cheese made of its milk, is forbidden as food. 
- The ancient Israelites looked with horror upon the custom prevalent among the surrounding nations of cutting off a limb or a piece of flesh from a living animal and eating it. Such limbs had to be thrown away.
- Water that was left uncovered overnight was not permitted as drink in olden times, because of the apprehension that a serpent might have left its venom in it. Where serpents are not found this prohibition does not exist.
Misconceptions about Kosher
Jewish people who do not keep kosher often complain that keeping kosher is difficult. Actually, keeping kosher is not particularly difficult in and of itself; what makes it difficult to keep kosher is the fact that the rest of the world does not do so. In fact, the basic underlying rules are fairly simple. If you buy only kosher certified products at the market, the only thing you need to think about is the separation of meat and dairy. Keeping kosher only becomes difficult when you try to eat in a non-kosher restaurant, or at the home of a person who does not keep kosher. In those situations, your lack of knowledge about your host's ingredients and food preparation techniques make it very difficult to keep kosher. Some commentators have pointed out, however, that this may well have been part of what God had in mind: to make it more difficult for us to socialize with those who do not share our religion. Products that have been certified as kosher are labeled with a mark called a hekhsher. Approximately 3/4 of all prepackaged foods have some kind of kosher certification, and most major brands have reliable Orthodox certification.The total number of certified kosher packaged food products in the United States has reached close to 100,000. The most controversial certification is the K, a plain letter K found on products asserted to be kosher. A letter of the alphabet cannot be trademarked, so any manufacturer can put a K on a product, even without any supervision at all. For example, Jell-O brand gelatin puts a K on its product, even though every reliable Orthodox authority agrees that Jell-O is not kosher.
Another misconception about Kosher is that a rabbi is required to bless food, which is false.
The reason why Kosher exists
According to AskMoses, a traditional Jewish website, "keeping kosher is the means of finding God in your food."
"With Shabbat, we can spiritualize time."
"With the Shul(synagogue), we can spiritualize place."
"With Tefillin, we can spiritualize leather."
"And with kosher, we can spiritualize breakfast/lunch/dinner/snacks."
In his classic book Think Jewish, Rabbi Zalman Posner eloquently elaborates the above point: most secular, Westernized Jews think in secular, Westernized terms. This results in the notion that spirituality is restricted to certain times, or certain places, or certain rituals, and that “real life” exists separately, outside spirituality.
But according to Judaism, spirituality is all times, and all places. Even when you eat.
Who buys Kosher certified products
$50 billion per year is spent on kosher foods by various different groups.
1. Jews seeking to meet their religious obligations 
3. Consumers insisting on a high-quality, safe and healthy product 
4. Lactose intolerant customers purchasing non-dairy (Pareve) Kosher products 
Kashrut in the New Testament
There is sufficient Scriptual evidence that in the New Testament Kashrut is abolished, or, at least, demoted from being a Divine imperative. This is primarily founded in the saying of Christ, "It is not what goes into a man that defiles but what goes out of a man that defiles him, for from the heart..." This is followed by either the words of the Gospel of Mark or slightly later gloss but representing the original apostolic understanding, "thus making all foods clean."
This understanding underpinned the new reality which the Church would not lose - from now on, the gentiles were grafted into the blessings of Abraham and in the Messiah were now one with the people of God, the Jewish believers in Jesus, and eat at the same table, partaking of the same food, and one with them in the most intimate and commonplace of all their activities - sharing around the dinner table. The common Eucharist, the Holy communion for all races of believers grew out of this understanding, and remained open to all, even, subsequent to New Testament times, after the actual meal portion dropped away. In the letter to the Galatians, Peter was taken to task by Paul for having violated this truth by absenting himself from the table attended by the gentiles.
Acts chapter 15 shows the ability to adapt to new situations and how the spirit of Jesus was applied with kindness, firmness, and understanding in the changing new situation of gentiles coming into the Church. The Old no longer had effect on the ingathering of the gentiles and they were free from the law of Moses. But they were not to eat meat with blood in it, a key element of Kashrut, for doing so was offensive to the Jews amongst them. "for Moses is read everywhere." - For the sake of peace. The unity of the Body of Christ was not to be violated.
Note: It may be that the prohibition against eating the blood came about from an understanding that the Divine intent in the original prohibition of the Old Testament was given as a pointer to the future sacrifice of the Son of God and the shedding of His blood. But this is not explicitly brought out in Acts 15. The rational in the original prohibition in the Old Testament reads, "For the life of the flesh is in the blood, and I have given it to you upon the altar to make atonement for the soul, for it is the blood which makes atonement for the soul." Lev. 17:11 Yet, the symbolic import of this interpretation is not foreign to Biblical interpretation, as the basis for not eating the portion containing the sciatic nerve is found in Scripture itself in the remembrance fostered by this restraint, of Jacob's struggle with the divine being and the subsequent wounding he received on the back of his thigh. One of the symptoms of sciatica is pain in the back portion of the thigh.
Use in popular culture
The term kosher has entered the popular culture to describe anything or anyone which is considered legitimate, honest or pleasing.
- Binyomen Forst, The Laws of Kashrus, Mozniam, 1999
- Isidore Grunfeld, The Jewish Dietary Laws, London: Soncino, 1972
- Isaac Klein, A Guide to Jewish Religious Practice, JTSA, 1992
- James M. Lebeau, The Jewish Dietary Laws: Sanctify Life, United Synagogue of Conservative Judaism, New York, 1983
- Yacov Lipschutz, Kashruth: A Comprehensive Background and Reference Guide to the Principles of Kahruth. New York:Mesorah Publications Ltd, 1989
- "Y2K" (2002) Committee For The Advancement of Torah. Retrieved 23 July 2008.
- "Kosher Terminology" Keeping Kosher: Jewish Dietary Laws. Retrieved 23 July 2008.
- "Kashrut: Jewish Dietary Laws" Judaism 101. Retrieved 23 July 2008.
- Shimoni, Giora "What is Kosher Food?" About.com: Kosher Food. Retrieved 23 July 2008.
- Jewish Encyclopedia's article on Dietary Laws
Resources on keeping kosher
- Online resource for keeping Kosher and becoming Kosher certified - The Orthodox Union
- Questions and Answers about specific food products and kashrut in general
- Guide to keeping kosher and Kosher Recipes
- Guide to keeping kosher
- International Kosher Symbols and Kosher Food Guidance
- Kosher Certification
- The Star K's website
- The O-K's website
- A useful guide to the issues of kashrus
- About Shechita: The Method of Animal Slaughter
- Controversy over recent PETA complaint
- Studies on pain during slaughter
- Conservative Judaism viewpoints on which methods should be considered kosher
- Proposed British ban on kosher and halal meat
- Campaign against all ritual slaughter
- About Kosher Food: Menus and Recipes
- Traditional Kosher foods recipes
- The Torah and vegetarianism
- Second Jewish Encyclopaedia's excerpt on kashrut and Jewish dietary laws
- Kashrut certification agencies (partial list) from around the world, and their symbols which are found on all food that has been deemed kosher.