Kosher means 'proper', referring to foods which are acceptable to be eaten by those of the Jewish faith who practice and observe certain dietary laws as prescribed in the Torah, the Old Testament. Such foods and food product derivatives are said to fall under the laws of Kashrut. These laws come primarily from the Bible, with additional Rabbinical decrees which have been handed down through generations.
Fish, eggs, vegetables, fruits and grain may be produced and manufactured with either milk or meat products. Thus, oils such as vegetable oils and shortenings may be used with both milk and meat products.
It is forbidden to eat or cook milk and meat together. Therefore, if any product includes both meat - even a kosher meat - and a dairy product in its ingredients can not be kosher. However, in today's highly sophisticated world of food technology many PARVE substitutes are available. It is also forbidden to use the same utensils that were used for manufacturing a non-kosher product for making a kosher product - unless the utensils or machinery are specially cleansed.
Only the meat of certain animals are kosher. These animals can be identified as having split hooves and chewing their cud - animals such as cows and sheep. There are many kosher animals such as deer, buffalo and others, however all animals must be slaughtered in a very specific ritualistic manner to be certified as kosher. Only a trained professional called a "shochet" may perform the slaughter.
Poultry & Other Fowl
We know which birds are kosher by means of tradition. The Torah lists which birds are not kosher. However, we are not sure of the exact translation of some of these species from the original Hebraic text in which the Bible was written. Therefore, only birds known by tradition to be kosher are considered so. This includes chicken, duck, turkey, quail, Cornish hens, doves/pigeons, geese, and pheasant.
All fish with scales and fins are kosher - fish such as tuna, carp, whitefish and salmon are kosher. All other seafood is not kosher such as all shellfish; shrimp, lobster, clams, oysters, scallops, and crustaceans (crabs, crayfish, etc.). Also, scavengers "bottom-feeders" such as catfish & monkfish are not kosher.
Fruits, Vegetables & Grains
All fruits, vegetables and grains are kosher. The one exception is with special laws pertaining to grape products.
Rabbinical supervision is required over all wines and grape products. This stems from medieval times when wine was used by non-Jews in pagan rituals and libations. These rituals rendered those wines unfit for consumption by Jewish people and these laws have held into modern times.
Because of this sacramental aspect of wine in Judaism, there are special laws governing all grape products and only those grape products which have proper supervision are considered to be kosher by observant Jews. This applies not only to grape wine but grape juice, grape jelly, vinegar, and all soft drinks that use white grape juice as a sweetener. It does not apply to fresh grapes or raisins.