The separation of milk and meat 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. Many a kosher household will have at least two sets of pots, pans and dishes: one for meat and one for dairy.
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, some customs say one must wait one full hour, and others that 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.
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.
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.
It is important to note that 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, many people who keep kosher and still wish to eat out will order cold food in a restraunt, to avoid problems with traife (un-clean) dishes.
Fruits, vegetables, and grains are called pareve - neutral. Pareve items can be cooked in and with both "fleishig" (meat) and "milchig" (dairy) foods and utensils.
Certain kinds of utensils can be "kashered" if you make a mistake and use it with both meat and dairy. The following may not be kashered:
- China, pottery, earthenware, rubber, non-stick and Teflon and similarly coated pots and pans, plastic dishes, enameled pots, pans and ladles, and utensils with wooden or plastic handles
- Utensils or vessels which cannot be thoroughly cleaned, such as those having crevices in which material can accumalate, (e.g. bottles with narrow necks, sieves etc.)
Plastics that can withstand the kashering process, may be kashered.
Kashering usually involves exposing the item to very high temperatures, with dry flame, boiling water, steam, etc. depending on the item.