Dumbledore is the beloved mentor of Harry Potter. He guided Harry, and gave him the necessary support during all the boy wizards trials and tribulations. His death was one of the most tragic in the series, and was a sign of Voldemort's impending vicotry over the wizarding world.
A recent fan theory has become popular regarding Dumbledore. This theory states that Dumbledore is actually Ron! Ron, when he is an elderly man, travels back in time to make sure things go as they need to, and gives Harry the necessary support.
What is the evidence? Well, Dumbledore states he had red hair when he was younger, much like Ron. In the books, Ron and Dumbledore are described as having the same nose, as well as a scar on the same leg. The two also have very little contact during the series. Ron is even surprised he is left something by Dumbledore after his death because of this fact.
Why wouldn't Dumbledore just tell Harry and his friends that he is future Ron? One reason could be he was afraid of messing with the timeline. This information could alter the future in major ways. Another reason is perhaps, as an adult, Ron learned something that would have been ruined if he told them.
J.K. Rowling has taken notice of the theory and addressed it directly. Her official stance on the theory is...
Oh well, it's still a fun theory. Just pretend Rowling never said that!