Here is an excerpt from a site which discusses the laws of Kashrut:
http://www.mechon-mamre.org/jewfaq/kashrut.htmThe Fundamental Rules
Although the details of kashrut are extensive, the laws all derive from a few fairly simple, straightforward rules:
1. Certain animals may not be eaten at all. This restriction includes the flesh, organs, eggs, and milk of the forbidden animals.
2. Of the animals that may be eaten, the birds and mammals must be slaughtered in accordance with Jewish law.
3. All blood must be drained from the meat or broiled out of it before it is eaten.
4. Certain parts of permitted animals may not be eaten.
5. Meat (the flesh of birds and mammals) cannot be eaten with dairy. Fish, eggs, fruits, vegetables, and grains can be eaten with either meat or dairy.
6. 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. This applies only where the contact occurred while the food was hot.
7. Grape products made by non-Jews may not be eaten or drunk.
The DetailsAnimals that may not be eaten
Of the "beasts of the earth" (which basically refers to land mammals with the exception of swarming rodents), you may eat any animal that has cloven hooves and chews its cud. Leviticus 11,3; Deuternomy 14,6. Any land mammal that does not have both of these qualities is forbidden. The Torah specifies that the camel, the rock badger, the hare and the pig are not kosher because each lacks one of these two qualifications. Sheep, cattle, goats, and deer are kosher.
Of the things that are in the waters, you may eat anything that has fins and scales. Leviticus 11,9; Deuteronomy 14,9. Thus, shellfish such as lobsters, oysters, shrimp, clams, and crabs are all forbidden. Fish like tuna, carp, salmon, and herring are all permitted.
For birds, the criteria are less clear. The Torah lists forbidden birds (Leviticus 11,13-19; Deuteronomy 14,11-18), but does not specify why these particular birds are forbidden. All of the birds on the list are birds of prey or scavengers, thus the rabbis inferred that this was the basis for the distinction. Other birds are permitted, such as chicken, geese, ducks, and turkeys.
Of the "winged swarming things" (winged insects), a few are specifically permitted (Leviticus 11,21).
Rodents, reptiles, amphibians, and insects (except as mentioned above) are all forbidden (Leviticus 11,29-30, 42-43).
Some authorities require a post-mortem examination of the lungs of cattle, to determine whether the lungs are free from adhesions. If the lungs are free from such adhesions, the animal is deemed "glatt" (that is, "smooth"). In certain circumstances, an animal can be kosher without being glatt; however, the stringency of keeping "glatt kosher" has become increasingly common in recent years.
As mentioned above, any product derived from these forbidden animals, such as their milk, eggs, fat, or organs, also cannot be eaten. Rennet, an enzyme used to harden cheese, is often obtained from non-kosher animals, thus kosher hard cheese can be difficult to find.
Kosher slaughtering
The mammals and birds that may be eaten must be slaughtered in accordance with Jewish law; "as I have commanded thee" (Deuteronomy 12,21) is according to the Oral Torah on kosher slaughter given to Moses at Sinai. We may not eat animals that died of natural causes (Deuteronomy 14,21) or that were killed by other animals (Exodus 22,30). In addition, the animal must have no disease or flaws in the organs at the time of slaughter. These restrictions do not apply to fish, which may be merely "gathered" (Numbers 11,22).
Ritual slaughter is known as shechitah, and the person who performs the slaughter is called a shochet, both from the Hebrew root Shin-Chet-Tet, meaning to slaughter. The method of slaughter is a quick, deep stroke across the throat with a perfectly sharp blade with no nicks or unevenness. This method is painless, causes unconsciousness within seconds, and is widely recognized as the most humane method of slaughter possible.
Another advantage of shechitah is that ensures rapid, complete draining of the blood, which is also necessary to render the meat kosher.
The shochet is not simply a butcher; he must be a pious man, well-trained in Jewish law, particularly as it relates to kashrut. In smaller, more remote communities, the rabbi and the shochet were often the same person.
Draining of Blood
The Torah prohibits consumption of blood. Leviticus 7,26-27; Leviticus 17,10-14. This is the only dietary law that has a reason specified in Torah: we do not eat blood because the life of the animal is contained in the blood. This applies only to the blood of birds and mammals, not to fish blood. Thus, it is necessary to remove all blood from the flesh of kosher animals.
The first step in this process occurs at the time of slaughter. As mentioned above, shechitah allows for rapid draining of most of the blood.
The remaining blood must be removed by salting, and then either broiling or emersing the salted meat in boiling water till it whitens. Liver may only be koshered by the broiling method, because it has so much blood in it and such complex blood vessels. This final process must be completed within 72 hours after slaughter, and before the meat is frozen or ground. Most butchers and all frozen food vendors take care of the salting for you, but you should always check this when you are buying someplace you are unfamiliar with.
An egg that contains a blood spot may not be eaten. This is not very common, but one finds them once in a while. It is a good idea to break an egg into a container and check it before you put it into a heated pan, because if you put a blood-stained egg into a heated pan, the pan becomes non-kosher.
Forbidden Fats and Nerves
The sciatic nerve and its adjoining blood vessels may not be eaten. The process of removing this nerve is time consuming and not very cost-effective, so most kosher slaughterers simply sell the hind quarters to non-kosher butchers.
A certain kind of fat, known as chelev, which surrounds the vital organs and the liver, may not be eaten. Kosher butchers remove this. Modern scientists have found biochemical differences between this type of fat and the permissible fat around the muscles and under the skin.
Separation of Meat and Dairy
On three separate occasions, the Torah tells us not to "boil a kid in its mother's milk" (Exodus 23,19; Exodus 34,26; Deuteronomy 14,21). The Oral Torah explains that this passage prohibits eating meat and dairy together. The rabbis extended this prohibition to include not eating milk and poultry together. It is, however, permissible to eat fish and dairy together, and it is quite common. It is also permissible to eat dairy and eggs together. According to some views, it is not permissible to eat meat and fish together, but we are not certain of the reason for that restriction (it has been attributed to medical opinion in the Middle Ages, for example).
This separation includes not only the foods themselves, but the utensils, pots and pans with which they are cooked, the plates and flatware from which they are eaten, the dishwashers or dishpans in which they are cleaned, and the towels on which they are dried. A kosher household will have at least two sets of pots, pans, and dishes: one for meat and one for dairy. See Utensils below for more details.
One must wait a significant amount of time between eating meat and dairy. Opinions differ, and vary from one or two to six hours. This is because fatty residues and meat particles tend to cling to the mouth. From dairy to meat, however, one need only rinse one's mouth and eat a neutral solid like bread, unless the dairy product in question is also of a type that tends to stick in the mouth.
The Yiddish words fleishig (meat), milchig (dairy), and pareve (neutral) are commonly used to describe food or utensils that fall into one of those categories.
Note that even the smallest quantity of dairy (or meat) in something renders it entirely dairy (or meat) for purposes of kashrut. For example, most margarines are dairy for kosher purposes, because they contain a small quantity of whey or other dairy products to give it a dairy-like taste. Animal fat is considered meat for purposes of kashrut. You should read the ingredients very carefully, even if the product is kosher-certified.
Utensils
Utensils (pots, pans, plates, flatware, etc., etc.) must also be kosher. A utensil picks up the kosher "status" (meat, dairy, pareve, or treyf) 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. Thus, if you cook chicken soup in a saucepan, the pan becomes meat. If you thereafter use the same saucepan to heat up some warm milk, the fleishig status of the pan is transmitted to the milk, and the milchig status of the milk is transmitted to the pan, making both the pan and the milk a forbidden mixture.
Kosher status can be transmitted from the food to the utensil or from the utensil to the food only in the presence of heat, thus if you are eating cold food in a non-kosher establishment, the condition of the plates is not an issue. Likewise, you could use the same knife to slice cold cuts and cheese, as long as you clean it in between, but this is not really a recommended procedure, because it increases the likelihood of mistakes.
Stove tops and sinks routinely become non-kosher utensils, because they routinely come in contact with both meat and dairy in the presence of heat. It is necessary, therefore, to use dishpans when cleaning dishes (do not soak them directly in the sink) and to use separate spoon rests and trivets when putting things down on the stove top.
Dishwashers are a kashrut problem. If you are going to use a dishwasher in a kosher home, you either need to have separate dish racks or you need to run the dishwasher in between meat and dairy loads.
You should use separate towels and pot holders for meat and dairy. Routine laundering koshers such items, so you can simply launder them between using them for meat and dairy.
Certain kinds of utensils can be "koshered" if you make a mistake and use it with both meat and dairy. Consult a rabbi for guidance if this situation occurs.
Grape Products
The restrictions on grape products derive from the laws against using products of idolatry. Wine was commonly used in the rituals of all ancient religions, and wine was routinely sanctified for pagan purposes while it was being processed. For this reason, use of wines and other grape products made by non-Jews was prohibited. (Whole grapes are not a problem, nor are whole grapes in fruit cocktail).
For the most part, this rule only affects wine and grape juice. This becomes a concern with many fruit drinks or fruit-flavored drinks, which are often sweetened with grape juice. You may also notice that it is virtually impossible to find kosher baking powder, because baking powder is made with cream of tartar, a by-product of wine making.
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