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:
And the Lord said to Moses and Aaron, "Say to the people of Israel, These are the living things which you may eat among all the beasts that are on the earth. Whatever parts the hoof and is cloven-footed and chews the cud among the animals, you may eat. Nevertheless among those that chew the cud or part the hoof, you shall not eat these: the camel, because it chews the cud but does not part the hoof, is unclean to you. And the rock badger, because it chews the cud but does not part the hoof, is unclean to you. And the hare, because it chews the cud but does not part the hoof, is unclean to you. And the swine, because it parts the hoof and is cloven-footed but does not chew the cud, is unclean to you. Of their flesh you shall not eat, and their carcasses you shall not touch; they are unclean 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.
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.
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.
Here are several interesting links if you would like to learn more: