Because very little about Tobi has been revealed, many can only speculate as to who's behind the orange swirl mask and bulky clothing, but many fans believe him to be none other than Uchiha Obito, the young boy from Kakashi Gaiden (chapters 239 to 244) who was a student of the Fourth Hokage and Hatake Kakashi's best friend. Sadly Obito did not live long enough to see his friends grow up, the sad fate of many young shinobi. Because this is about the theory of the man behind the mask, I'll leave Obito's story out. However, you can go visit Wikipedia's entry on Obito. While its not detailed, it should provide enough information about Obito to help you.

There seems to be a heavy amount of evidence supporting that Tobi might very well be this young man, even though he's passed on. However, can be noted that despite his injuries, we never did see Obito truly die - we only saw his last wishes until the rocks above crumbled down and began to bury him. Some argue that Obito could not have survived due to the large rock that crush his right side. Of course that's a possibility, but we must not forget that one Akatsuki member could have the power to save his life. The organization has also been around long enough to provide this as according to the manga, Orochimaru had joined the organization about 13 years prior to the series. He is well known for his horrific experiements and could have found a way to save Obito and turn him to Akatsuki.

If you reverse the katakana of Obito's name, it becomes 'Tobi'. Another piece of evidence is the anogrym of Obito's name; if you drop the first 'O' and reverse the 'bi' and 'to', you get, 'Tobi'. Why would Kishimoto name a character as such with little reason to name him so close to a fallen shinobi?

Another is that in Kakashi Gaiden, Obito was buried by earth and rock within the bountries of Grass Country after his and Kakashi's battle. Ironically in the beginning of Part II, Tobi was seen working as an accociate of Zetsu who hails from none other than the Grass Country.

The most obvious and key piece of evidence is none other than the spiral mask Tobi dones. It covers most of his face except one eye - the eye that Obito would have remaning after asking Rin to implant his left eye to his friend Kakashi, giving the said ninja the ability to use Sharingan. Not only that, but Tobi actually does possess the Sharingan in the eye that would have been left to Obito.

Whether personality is a true arguement for this topic is still an issue, but never the less both act fairly similar; both are clumsy characters, goofy and carefree. In addition, if Obito had survived the falling rocks, he would no doubt be the same age as Kakashi. Tobi seems to be around the same height as Zetsu without the venus fly trap surrounding his body, so it's safe to assume that Tobi is in the age range of early 20's to mid 30's.

While some of these points can be considered a coincidence, why would so many coincidences surround one man?