What do kosher and halal mean, and what are the differences between them?
Linguistically, at least, they’re very similar. Halal is Arabic for “permissible,” and kosher is Hebrew for “fit” or “proper.” Both refer to anything—not just food—that is allowed or correct under Islamic or Jewish religious laws, respectively. The opposite of halal is haram, meaning “forbidden,” and the opposite of kosher is treif, meaning “torn” and referring to the fact that flesh torn from an animal by wild beasts is not kosher.
Kosher and halal rules both prohibit consuming pork or animal blood of any kind. To be kosher or halal, a land animal or bird must be slaughtered by cutting the throat with a single stroke without cutting the spinal cord. All of the animal’s blood must also be drained completely (kosher salt is so named because the large crystals are good for drawing any residual blood out of meat). Both sets of rules also prohibit birds of prey and allow any other kind of bird. To be kosher, a land animal must have cloven hooves and chew cud (sheep, cattle, goats, deer, and bison are essentially the only kosher animals), but anything that’s not a pig or a carnivore can be halal.
Under kosher rules, a permissible fish must have fins and scales, and cannot be a scavenger or bottom-feeder (wild catfish are treif, but some farm-raised ones can be certified kosher, since their diets are controlled). There is some disagreement among halal authorities about fish. All kosher fish are definitely halal, but some interpretations allow the eating of shellfish. Both sets of rules forbid eating frogs and other amphibians.
As for processed foods, kosher and halal certifications (the most common are the Orthodox Union’s U_ inside an _O for kosher and the Islamic Food and Nutrition Council of America’s M inside a crescent for halal) verify that no substances derived from forbidden animals are contained in the product, and that it wasn’t made using machines that process treif or haram foods. Some kosher authorities allow animal-derived gelatin because it has been changed so much in processing that it is no longer considered meat, but most halal authorities do not.
The biggest difference between the two is that kosher rules forbid the mixing of meat and dairy products. People who keep strict kosher have two sets of dishes and sometimes two refrigerators to keep them separate (kosher products labeled pareve are neither meat nor dairy and can be eaten anytime). Under halal rules, though, cheeseburgers are totally OK. Islam bans alcohol, so products containing even a tiny amount are haram. Judaism is much more booze friendly—all distilled liquor and most beer is kosher, and wine (and all other grape-based foods) can be kosher if made only by Jews.
Many strict halal butchers and restaurants call their food zabihah halal. The word refers to the process of halal slaughter and generally means that special care has been taken to ensure that all the slaughtering rules have been followed. Glatt kosher is the equivalent on the other side. Glatt means “smooth,” and means that the animal’s lungs have been examined for blemishes (diseased animals are treif), but it has come to refer to any strict level of kosher adherence. Zabihah.com provides a directory of halal restaurants and markets around the world, and the Orthodox Union lets you search the thousands of products it certifies as kosher.