On three separate occasions, the Torah tells us not to "boil a kid in its mother's milk." (Ex. 23:19; Ex. 34:26; Deut. 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. In addition, the Talmud prohibits cooking meat and fish together or serving them on the same plates, because it is considered to be unhealthy. 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.
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 three 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 fleishik (meat), milchik (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.
