A product receives a kosher status if a specially trained rabbi has inspected the facilities that the product is made and packaged in and certified that it meets the necessary requirements. These rabbis usually belong to specific groups, there are several of them. The initials or symbols you see indicate which groups have inspected that particular product. In this case, "OU" is one of the major ones and you'll see it as either "OU" or an O with a U inside of it. The added "D" means that it's considered a dairy product and therefore cannot be eaten with meat (or put on the same plate as meat if you're being strict).
Other kosher trademarks are Circle-K, CRC, Star K, etc. They all have their own followings and many people will ONLY buy products with the particular mark that they trust.
Here are a bunch of the different agencies' logos and links to info about each one.