Kosher is the Hebrew word that refers to a strict, complex set of dietary laws adhered to by Orthodox Jews.
Kosher is commonly and incorrectly understood as "the rabbi's blessing over food," which is said to somehow make it permissible to the Orthodox Jewish palate. In fact, the laws of kosher trace back many centuries to the Talmud, a medieval work of Jewish law and lore that first outlined the full complexity of the concept called kosher. Firstly, certain meats such as pork may not be consumed, as the pig is deemed by the Bible to be ritually unclean. Permissible meats must first be produced under strict slaughtering conditions, supervised and/or performed by a competent rabbi. A specialized, super-sharp knife is used to instantly sever the animal's throat, rendering it immediately brain-dead. If there is so much as a miniscule nick on the blade of this knife, the animal is deemed unkosher and forbidden to eat.
Next, the animal's organs are examined for signs of decay and disease, which would also render it unfit if such a thing were to be found. Finally, the meat must be salted and thus cleansed of all blood, as the consumption of blood is forbidden under the laws of kosher. Salting may also be performed by the competent consumer if he/she wishes.
Kosher is also used to refer to the prohibition of consuming meat and dairy products together. In addition to this, consumers of meat are required to wait between three and six hours before eating dairy to ensure the complete digestion of the meat. (Vice-versa, there is no such law.)
By extension, Orthodox Jews usually keep two entirely different sets of dishes--one set for all meat meals and one for dairy. Some will even obtain a third set for meals that are neither meat nor dairy. All pains are taken to ensure that particles of--or even steam from--a dairy or meat dish does not come in contact with the opposing food. If such a blending were to occur, the food would then be deemed unkosher.
The spiritual meaning behind the laws of kosher is an understanding that food is meant to sustain the believing Jew and give him strength to serve God; it is not an end in and of itself. By limiting and refining the foods that enter one's body, one may lend the simple act of eating a spiritual focus beyond gluttony.