From Chabad.org (http://www.chabad.org/kosher)
The Hebrew word kosher means "fit." The kosher laws define the foods that are fit for consumption for a Jew.
The kosher laws were commanded by G-d to the Children of Israel in the Sinai desert.Moses taught them to the people and wrote the basics of these laws in Leviticus 11 and Deuteronomy 14; the details and particulars were handed down through the generations and eventually written down in the Mishnah and Talmud. To these were added various ordinances enacted through the generations by the rabbinical authorities as "safeguards" for the biblical laws.
Throughout our 4000-year history, the observance of kosher has been a hallmark of Jewish identity. Perhaps more than any other "mitzvah," the kosher laws emphasize that Judaism is much more than a "religion" in the conventional sense of the word. To the Jew, holiness is not confined to holy places and times outside the everyday; rather, life in its totality is a sacred endeavor. Even the seemingly mundane activity of eating is a G-dly act and a uniquely Jewish experience.
- The meat, milk and eggs of certain species of animal are permitted for consumption, while others are forbidden. In addition, a series of laws govern how the animal should be killed and which parts of the animal can be eaten.
- Meat and milk are never combined. Separate utensils are used for each, and a waiting period is observed between eating them.
- Fruits, vegetables and grains are basically always kosher, but must be insect free. Wine or grape juice, however, must be certified kosher.
- Since even a small trace of a non-kosher substance can render a food not kosher, all processed foods and eating establishments require certification by a reliable rabbi or kashrut supervision agency.