Kosher is a set of religious rules that dominate nutrition observant of the Jewish people. The word "kosher" means in accordance with the law, right, permitted.
Kosher laws (kashrut) are derived from the Bible, and are detailed in Talmund (one of the sacred texts of Judaism) who, along with other codes of Jewish traditions, is considered the transmission of Torah. The Bible lists the categories of foods that are not kosher, including some animals, birds and fish (such as pork and rabbit, eagle and owl, catfish and sturgeon), most insects or crustacean or reptile.
Milk and dairy products, of any kosher animal are themselves kosher and "milk". Because you can not distinguish kosher milk (ie a kosher animal) from non-kosher, the rabbis decreed that it must be controlled from the milking to packaging, to ensure that it comes from a kosher animal. In many countries of the world in which the origin of the milk on the market is guaranteed by law, some rabbinical authorities had at their time claimed that the milk is guaranteed as kosher and therefore must not be controlled.