80% of Kosher Products users are non-jewish :)
Kosher foods are those that conform to the regulations of the Jewish Halakhic framework. These rules form the main aspect of kashrut, Jewish dietary laws.
A list of some Kosher foods are found in the book of Leviticus. There are also certain Kosher rules which are found there.
Reasons for food being non-kosher include the presence of ingredients derived from non-kosher animals or from kosher animals that were not properly slaughtered, a mixture of meat and milk, wine or grape juice (or their derivatives) produced without supervision, the use of produce from Israel that has not been tithed, or even the use of cooking utensils and machinery which had previously been used for non-kosher food. These might include utensils and machines used for making pork or other non-kosher foods.