In the grocery store you will find all sorts of foods labeled as Kosher, but the most common are Kosher hot dogs and Kosher pickles. Heinz Ketchup has been Kosher for decades. You will often find a U with a circle around it on Kosher products. There is also the word Pareve or Parve on many foods, and this is a form of Kosher labeling as well.
When a food is labeled as Kosher, it means that the food has been prepared in accordance with the rules of food preparation set forth in the old testament of the Bible and formalized in Jewish law. For example, in Chapter 11 of the book of Leviticus, the Bible states:
These you may eat, of all that are in the waters. Everything in the waters that has fins and scales, whether in the seas or in the rivers, you may eat. But anything in the seas or rivers that has not fins and scales, of the swarming creatures in the waters and of the living creatures that are in the waters, is an abomination to you.
It goes on to cover birds, insects, and so on in quite a bit of detail. Other parts of the Bible indicate that the mixing of meat and milk is forbidden, that animals shall be killed humanely according to certain rituals, and so on. By following all of these different rules under rabbinical supervision, a food can be certified and labeled as Kosher.
Meat and milk can not be mixed, they cannot be eaten at the same time, and even the utensils used to prepare meat and dairy foods must be separated. On the other hand, most vegetables, fruits, grains and nuts can be mixed with either meat or milk. Therefore, these foods can be consumed at any time. A food that can be eaten either with meat or milk is labeled as pareve. This label indicates that it is safe to eat the food at any time.
To learn more about kosher foods, check out the links on the page.