OK, help out a clueless goy.
I know that a circled “U” on a food product means that a certain board has certified it kosher.
I saw on a bag of “Snyder’s of Hanover” pretzels the familiar circled “U” with the word “Dairy” underneath it.
What is the relevance of the “dairy” in connection with the circled “U?” I guess it’s possible that it has nothing to do with it, and that it’s just coincidentally close.
It means, as you might guess, that the product contains dairy products. You see, if you want to keep kosher, you’re not supposed to eat dairy products and meat products together in the same meal. So, if you want to eat the pretzels with your roast beef, you’re out of luck. However, if you want to finish off the pretzels with a big glass of milk, that’s ok.
Chances are those pretzels were made in a factory that processes dairy foods or some of the preservatives used are dairy based. I’m not Jewish, but I understand that those laws are pretty stringently enforced. Remember, just one little drop of milk is enough to render literally tons of food “dairy”, even though there may not be any measurable dairy in the finished product. That’s why you will often see the little (U) on sandwich bags, sink cleaners, etc. It may seem overly anal to you, but that’s all a matter of perspective. 
Very often, non-food items will be made from substances that may have animal origins. Therefore, these items - even though we don’t eat them - have to be certified as well.
Also, food that is processed on equipment that was used to make dairy foods is considered dairy. For example, suppose the pretzel factory made a batch of pretzels that had milk in them, then made a batch of pretzels on the same equipment without milk. These pretzels would also be considered dairy.
It used to be that the OU would either label a product as dairy or DE (dairy equipment), but there was too much confusion. From that point on, all items made on dairy equipment were considered as dairy.
Here’s a link to Kosher definitions and a list of symbols used by one certifying company: http://kosher.org.
Click on “What is Kosher?” and “Kosher Symbols,” which includes a short definition and use of each symbol.
Since the symbols are copyrighted, different companies use different designs, but they have considerable similarity.
Mazeltov!
All kosher foods are categorized as “Dairy”, “Meat”, or “Pareve” (neutral.)
Obviously, for some items (like a package of corned beef, say), it’s pretty clear that’s meat. Similarly, cheese is pretty clearly milk. But there are are items where it might not be so clear – such as, pretzels. They could be made with some animal product, they could be made with milk in the dough, or they could be neutral (neither milk nor meat, and able to be used with either.) Hence, the label may specify “Dairy” or “Pareve” (sometimes just a “P” for the latter) for clarity.
Frinstance, it’s now possible to have a pareve cheeseburger, using non-dairy cheese and soy-burger.
Another reason for the Dairy designation is that there are many Orthodox Jews who use milk that relies on a certain well-known halachic ruling. There are others who do not. The OU organazation will certify products relying on this ruling, and many people will not use products that are marked OUD for this reason.
This is not correct. The “P” means that the item is kosher for use on Passover.