I see these symbols on food labels a lot. If you're not Jewish yourself, or don't have Jewish friends, than you probably didn't know that the symbols mean that the food is kosher.
Contrary to popular misconception, rabbis or other religious officials do not "bless" food to make it kosher. Basically, specific dietary laws are written in the Torah (the Jewish teachings) and that determines what is kosher and what is not. Rabbis need to supervise to make sure these laws are strictly followed in order to be labeled kosher.
Many of the laws have to do with animals, which ones can be eaten, the way they are slaughtered, and the way they are prepared. In the Torah, it says something about "not boiling a kid in its mother's milk, the "kid" being a baby goat. So that's why people who are kosher don't eat milk and meat together.
The reason there are different symbols is to show where the food was supervised, and by which rabbi. For example, the Capital "K" with a circle around it was supervised in Brooklyn, NY by Rabbi Don Yoel Levy.
The symbol with the Capital "U" and the circle or "O" around it is the symbol of the Orthodox Union, a leader in kosher supervision.
The one with the Star of David is kind of a universal kosher symbol. Inside it are Hebrew letters that make up the word "kosher."
If you find a kosher symbol that says "parve," it means that the food contains no meat or dairy products.
Some food will say "kosher for Passover." Passover is actually coming up (April 2, 2007), so you might be seeing this symbol a lot.
Want to know what kosher for Passover means? Then read more
For the week of Passover, there are stricter rules and guidelines. Jews are instructed not to eat any foods that contain "chometz" which means no wheat, spelt, barley, oats or rye. Sounds like a celiac's diet.
Strict or Ashkenazi Jews won't eat rice, corn, soy, string beans, peas, lentils, peanuts, mustard, sesame seeds, or poppy seeds either.
Passover only lasts 7 days, so Jewish people can start eating these foods again after the week is up.