An urban legend is a folktale that has a much more recent origin (Industrial Revolution and newer) than traditional folklore. These stories are often believed to be true by those telling them, and often use the "friend of a friend" story device to lend authenticity to the tale. They are used in much the same way as folklore and legends have always been used—as cautionary tales, cultural validation and transmission, entertainment, and wish fulfillment/examples of a better way of doing things.
Urban legends are sometimes encountered by the PPC, whether because they have been made into movies, like Sweeney Todd, or because they have been summoned by foolish Sues—the Slender Man and Bloody Mary have been mentioned as possible summonees—or because a PPC agent wants to use the legend in question as an execution method, such as the Holder Series.