I would like to know the meaning and origin of the phrase "yellow journalism"-- I have an idea that has to do with sensationalist reporting of an event which has dubious credibility, but I am not sure.

I found this for you.

The Sensational Beginnings of Yellow Journalism

In 1898, newspapers provided the major source of news in America. At this time, it was common practice for a newspaper to report the editor's interpretation of the news rather than objective journalism. If the information reported was inaccurate or biased, the American public had little means for verification. With this sort of influence, the newspapers wielded much political power. In order to increase circulation, the publishers of these papers often exploited their position by sponsoring a flamboyant and irresponsible approach to news reporting that became known as "yellow journalism." Though the term was originally coined to describe the journalistic practices of Joseph Pulitzer, William Randolph Hearst proved himself worthy of the title. Today, it is his name that is synonymous with "yellow journalism."
http://tnt.turner.com/movies/tntoriginals/roughriders/jour.home.html

Around the turn of the century newspapers had vast political power because they could publish flamboyant, unobjective stories. The most famous culprits of this were Joseph Pulitzer and William Randolf Hearst. One of the features of Pulitzer's New York World newspaper was a political cartoon called "The Yellow Kid" by R.F. Outcault. The term "yellow journalism" mainly refers to the content of the newspaper itself and not the cartoon, but that is where the name came from.