Paul McCartney, as a member of the Beatles during the 1960s and as a solo musician since, is a singer/songwriter who needs no introduction. Everybody knows who he is and what he has accomplished in popular music and culture during the past forty-five plus years.
Those who are younger, however, probably haven't heard about the "Paul is dead" hoax of the late 1960s, just around the time that the Beatles were breaking up. According to the hoax, McCartney apparently was killed in an auto accident around the time that the Beatles stopped touring in 1966, and that the guy who has been "Paul" ever since is just an imposter. Supposedly, various clues in the Beatles' music and on their album covers from 1966 onward revealed "the truth" about McCartney's fate.
Now I think that the people who buy into this rubbish that the "real Paul McCartney" died back in 1966 are a bunch of kooks, but nonetheless, it's pretty amazing to see what people have come up with to support their belief in this hoax. Start here (and don't worry, you're not going to get Rickrolled) and watch all the parts of this documentary that dissects the "Paul is dead" phenomenon.