Seen on the packaging of certain foods, the symbol consisting of the letter "U" inside of a circle means what?

The most common symbol of kosher products is the capital letter "U" inside of a circle, which is popularly known as the "OU" sign. This registered certification mark appears on food packaging and stands for the Kosher Certification Service of The Union of Orthodox Jewish Congregations of America, (known as Union of Orthodox or Orthodox Union Kosher, and also known as OU Kosher or OUK for short), indicating the products have been processed according to Jewish dietary laws.

This marking can be found on the packaging of Oreos, Coca-Cola, Heinz, and many other world-known brands. The small letters next to the kosher symbol indicate if the food contains dairy, meat, pareve (neither dairy nor meat), or kosher for Passover.