Tobi is the newest member of Akatsuki and replaced Sasori upon his death at the hands of Team 7. Tobi's true identity is only known to a very select few, Kabuto and Zetsu. Most of the main characters that knew of his secret are dead.
Tobi is very care-free and goofy unlike the other members of Akatsuki who are very serious. Some members dislike his perspective on life and can get annoyed by Tobi such as his partner Deidara who thinks of him as a pet, where as Kisame and Zetsu appreciate Tobi's ability to brighten up a gloomy organization. In episode 139, our perspectives on the light-hearted and cheery character known as Tobi would change, as he claims himself to be a very old and powerful foe. He pledges allegiance with Team Hebi (hawk) and their ultimate goal is to capture all 9 tailed beasts and form them into the legendary 10 tailed beast, which has the power to render the worlds powers useless.
His true identity, as revealed in Chapter 599, is that of Obito Uchiha, Kakashi's best friend and the person that he got his Sharingan from.
Tobi has displayed a few jutsu thus far, such as an unnamed teleporting jutsu which he can use to teleport himself or others, a related unnamed phasing jutsu, an unnamed katon, Mole Hiding. He possess many unique jutsu that only he knows, Including the ability to transport himself, parts of his body and others to different dimensions.
He has also shown the ability to create replacement body parts. He also gained possession of Hashirama's power and learned to use Izanagi, allowing him to turn fantasy into reality.
He has recently gained the Rinnegan. Aside from granting it's user all 5 elements, it possesses 6 abilities based on the 6 paths of Buddhist reincarnation: gravity manipulation (Deva), body modification (Asura), telepathy (Human), summoning (Animal), chakra absorption (Hungry Ghost), and resurrection (Hell).