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Stemming from initial prohibitions on eating blood (Deuteronomy 12:20-28), ritually unclean animals, the flesh surrounding the sciatic nerve (Genesis 32:32), and a somewhat cryptic passage that states not to "boil a kid in its mother's milk"[note 1], kashrut developed in later, post-Torah tradition to include many of the following rules:
- Land animals may only be eaten if they chew the cud and have a cloven hoof (horses and pigs are notably banned). Carnivorous mammals and wild game which has not been slaughtered according to Jewish law are also banned. Giraffes are kosher, but good luck getting one to hold still while you kill it properly.
- Only fish with fins and scales may be eaten; catfish, shellfish, reptiles, amphibians, crustaceans, and insects are not allowed (except for a very specific type of locust). Incidentally honey is allowed though the bees that produce honey are forbidden, after all God promised Moses a land flowing with milk and honey, so milk and honey must be allowed.
- Birds must not be carrion-eaters or birds of prey. Mixing poultry with milk was banned by the rabbis because of the similarity in the way poultry and meat are slaughtered and eaten. Turkeys were only kosher by accident; everyone assumed they were kosher before someone realized they weren't in the torah, but by then it was too late and, tradition.
- Meat and milk may not be mixed in the kitchen or on the table. (However, fish, eggs, and plant-based foods are known as pareve, meaning "neither", and may be eaten freely with either milk or meat.[note 2]) Some observant Jews will have special meat or dairy plates, dishwashers, sinks, or even refrigerators. However, most Jews have historically lived in communities where things like "separate refrigerators" weren't a thing, and cheeses and smoked meats tended to both be stored in the same cellar. This meant accidents were bound to happen, and as long as a mixing was an accident and the mixing was less than 1 in 60, the food was still kosher. This does not mean that the Ultra Orthodox actually pay attention to their own traditions. This supposed prohibition is a case of biblical hyper-metaphoricism, since the original verse that this is all based upon is Deuteronomy 14:21-20
"Thou shalt not seethe a kid in his mother's milk". The difference between this one sentence and a ban on consuming ANY animal meat with ANY dairy product within several hours of one another, plus the excessive precautions shown above, is rather drastic.
- Mammals must be killed with a single stroke of a well sharpened knife to the neck and drained of blood; animals not killed in this manner are not acceptable, nor are animals that are diseased or damaged. Birds like chicken were originally exempt from this rule, but because they were processed by the same workers as the goats, they also ended up with this rule.
- The sciatic nerve is forbidden; some Kosher butchers remove this, but most sell the back half of the animal to a non-Kosher butcher. Don't expect to find sirloin or filet mignon at kosher restaurants.
- During Passover, a few more complicated regulations exist, forbidding leavened bread, beer, or any other wheat, spelt, rye or oat products other than matzah. Rice and corn are generally up for debate—Sephardim are fine with them, Ashkenazim usually are not[note 3]. During Passover, you should definitely stock up on Kosher Coke, because it's sweetened with cane or beet sugar instead of high fructose corn syrup, which causes near instant diabetes, irrepressible anxiety, makes your tongue feel like a piece of shoe leather, and will damn you to an eternity of servicing BDSM fetishes in hell, you fucking goy heathen! (This recommendation applies primarily or exclusively to American Jews; HFCS is only used in Coca-Cola in the United States due to tariffs on sugar and economics elsewhere favor the use of cane sugar.)
- Kosher salt (more accurately known as koshering salt) is not named because it is kosher in contrast to other forms of salt[note 4], but for its role in rendering meat kosher by soaking up the blood inside. The large flakes of kosher salt are more effective for this purpose than more common small-grained salt.
- Plants, although generally considered pareve, are not considered such when they are infested with insects or animals with "very short legs". However, "it is economically impractical to grow, harvest, or process raw products that are totally free of non-hazardous, naturally occurring, unavoidable defects". Similarly, this makes it impossible for someone to be a "pure" vegan. Generally, it's never been an issue so long as the person gets rid of the obvious bugs, you know, advice like "wash your hands before you eat" or "bury your dead" that primitive communities would figure out and ritualize even if they didn't understand why. However, a number of Ultra-Orthodox refuse to touch leafy vegetables like cabbage and lettuce unless it's been specially prepared. Even though literally no one gave a shit until 2 decades ago. 
Kashrut is rejected by almost all branches of Christianity, although a few sects, such as Seventh-day Adventism, maintain kosher dietary requirements, along with other Jewish traditions. Halal, the dietary strictures of Islam, is heavily modeled on kashrut but is less strict. With the exception of wines and other alcoholic products, anything kosher is halal, but for... reasons, a number of Imams in recent years have been declaring otherwise.
For the most part, kosher regulations are highly complex and differently enforced depending on the community; on the whole, Sephardic kitchens are less strict than Ashkenazic, and some Reform and Reconstructionist Jews obey kashrut only during Passover.
Kosher in slang
In common parlance, "kosher" is used to mean "okay" or "cool" - as in "don't tase me, bro — it's not kosher!" It is also used to mean things being legal or legitimate — as in "keeping it kosher".
According to conspiracy theorists, if you buy food that is kosher certified, part of the purchase price is a "kosher tax" going to fund some international Jewish conspiracy. This is based on the fact that food companies have to pay for kosher certification. Proponents of the idea include Canadian politician and feminist Louise Mailloux. However, kosher certification is not a big money raiser: in 1975, Kraft reported that certification cost .0000065 cents per item; the payment goes to fund the complex series of checks needed to assess modern manufacturing techniques.
- Usenet rec.food.cuisine.jewish Kosher FAQ
- Roden, Claudia, The Book of Jewish Food, New York: Knopf, 1996, ISBN 0394532589.
- This was a regional dish. Some Israelites found this barbaric, obviously.
- In practice, kosher kitchens keep separate sets of cookware and plates, and kosher restaurants are traditionally either dairy or flayshig (meat). So, no, you can't get a cheeseburger in an Israeli McDonald's.
- Not like they would buy sake or bourbon anyways.
- Which didn't stop one enterprising Evangelical from marketing "Christian salt"
- Kosher Food Certification: Does Kosher Mean?
- FDA: The Food Defect Action Levels Levels of natural or unavoidable defects in foods that present no health hazards for humans
- Kosher Lettuce
- See the Wikipedia article on Kosher tax conspiracy theory.
- Marois defends PQ candidate accused of anti-Semitic beliefs, Toronto Globe and Mail, Mar 14, 2014
- Calling It Kosher: How to and Why, New York Times, May 18, 1975