An Urban Legend is usually a (captivating/titillating/
                          engrossing/incredible/worrying) story that has had a 
                          wide audience, is circulated spontaneously, has been 
                          told in several forms, and which many have chosen to 
                          believe (whether actively or passively) despite the 
                          lack of actual evidence to substantiate the story.

                          An Urban Legend can be based on a true story. In these 
                          circumstances, what makes the story an Urban Legend is 
                          how it's told -- i.e., being told in the first, second 
                          or third person (or, as having happened to me, to my 
                          friend or to my friend's friend) when in fact the true 
                          event took place to someone entirely unknown to the 
                          person relating the story.

                          An Urban Legend ordinarily has a developed storyline. 
                          This means that it is usually transmitted in a form 
                          that sounds like a story a series of incredible events 
                          that took place somewhere to someone.