What's with the whole Tobi/Madara/Obito thing? ((spoliers!!))?

Okay, I know a LOT about Naruto as it is, but what I don't understand is this relationship between all these "Characters". I know about Obito being Kakashi's former teammate and the fact that when half his body was crushed by a boulder, he offered Kakashi his only working Sharingan eye, which he took. Supposedly, Obito died. I know that Madara was an Uchiha that went up against the first Hokage at the valley of the end, and that he might have been the one to summon the nine tailed fox on Konoha, and I know that Tobi is known little about. I've heard theories from "Tobi IS madara" to "Madara found Obito's body, took over it, and renamed himself Tobi" to "Tobi is Obito." but I'm confused. Everything seems weaved together some how, but I can't figure out how it is. Also, it is now apparent that Tobi is the true leader of Akatsuki, but what would "Madara" or "Obito" or both want with it? Would someone please help me solve this issue that 's been driving me mad lately? Thanks much ^^