What is Kosher?

The definition of kosher is usually explained as acceptable or proper. It has informally been used in the English language as that meaning. The laws of kosher food originated in the Bible, and have been observed by Jews for over 3,000 years. These laws are detailed in the Talmud and other codes of Jewish tradition. The laws of kosher go beyond the prohibition of not eating pigs. There are many intricacies involved in the basic kosher laws.
The Bible lists the basic categories that are not kosher Meat, fowl, fish, most insects, and any shellfish or reptile (Pig, camel, eagle, and catfish etc.). The animals that are permissible to eat must be slaughtered according to Jewish law. Meat and dairy items must be kept separate and not manufactured or consumed together. In order to indicate the variables of kosher and non-kosher depends on the source of the ingredients and the status of the production. Kosher certification guarantees that the food meets kosher requirements.

The guidelines for the sources of kosher and non-kosher materials originate in the Bible.  Kosher food is separated into three different categories, Meat, Dairy and Pareve.

Meat:
Mammals that chew their cud and have split hooves are deemed kosher. They must have both of these characteristics in order to be rendered kosher for example: cow, deer, and goat. After determining the status of the animal, they must be slaughtered by a specialist (shochet) and then koshered (soaked and salted to remove the blood) before eating. All other animals, even if they have one of these features are not considered Kosher, for example: Pig, camel and rabbit.
Fowl:
All non kosher fowl is listed in the Bible in Deuteronomy. Fowl falls under the meat category.  Examples of kosher fowl are chicken, duck, and turkey.

Of all of the animals and fowl that are permitted to eat, they must be killed in accordance to Jewish law.

Dairy:
Dairy foods must come from a kosher animal. They may not be eaten together with meat or fowl.

Pareve:
Pareve foods are neither meat nor dairy. Examples are vegetables, fruit, grains they must be in their natural state to be determined as Pareve. Fish which must have fins and scales is also considered pareve. Examples of kosher fish are tuna, salmon, tilapia. All shellfish, shark, reptiles and underwater mammals are not kosher.

Equipment used to manufacture non kosher ingredients may acquire a non kosher status. Therefore, production done after a non kosher run may render the food non kosher even if only kosher ingredients were used. Non kosher equipment can be restored to a kosher status by koshering the equipment. This can be done by either pouring hot water over the equipment or raising the heat on the oven depending on the equipment and what non kosher item was used on the equipment beforehand. Kosher food must adhere to the rules listed above. Jewish foods for example knishes, latkes, kugels etc.  are not kosher unless prepared in accordance to the laws of Kashrus. There are numerous rules and regulations regarding Kosher certification. In recent years, different Kashrus organizations certify foods on the food package, in order to determine their kashrus status.  This is usually indicated by a symbol stating its kashrus status.  For example, OU, OK, Kof-k, and star K. These organizations send emissaries to the food plants to ensure that the product compiles with the appropriate kashrus standards. 

There are many more intricacies involved in the laws of kosher. Please check out the following websites for more information about kosher.
http://www.ok.org/Content.asp?ID=22
http://www.jewfaq.org/kashrut.htm
http://www.oukosher.org/index.php/basics/what
http://www.kosher-directory.com/meaning.htm