Ask an average person to describe kosher food and they might say it is food "blessed by a rabbi." The word "kosher," however, is Hebrew for "fit" or "appropriate" and describes the food that is suitable for a Jew to eat. With its roots in the Hebrew Bible, the system of defining which foods are kosher was developed by the rabbis of late antiquity. Its application to changing realities has been the work of subsequent generations, including our own.
Close readers of the Torah might notice that according to the book of Genesis, vegetarianism was commanded by God as the ideal diet (see Genesis 1:29). However, in the course of the biblical narratives, this changed to include a variety of different animals. According to the Torah (Leviticus, chapter 11), only certain kinds of animals are considered inherently kosher. For land animals, any creature that both chews its cud and has split hooves is kosher. For sea creatures, any fish that has both fins and scales is acceptable, and for birds, only those birds approved by the Torah (or others that later authorities have judged to be like them, a list that excludes scavengers and birds of prey). In addition, it is repeated three times in the Torah that it is forbidden to cook a baby goat in its own mother's milk.
The rabbis in the Talmud further developed these principles of kashrut. In order to consume kosher land animals and birds, it is necessary to slaughter them in a prescribed way, in a manner that has been described as a more humane method than is practiced commercially. In addition, the prohibition of cooking a baby goat in its own mother's milk is the basis for the complete, physical, hermetic separation of all milk and meat products. These are the fundamental elements of kashrut.