Okay, Ive been asked to explain what's happened to me over the last 12 years, so I'll take my best shot at it. I'm gonna do this in little bits, so don't expect a full, long-winded explanation out of me right away.
Anyway, after I'd sealed the Kyuubi Demon into the boy, Naruto, I was gravely weakened. I'd shared the "Life Burden" with Naruto, so neither of us died, but both of us were severely weak, for a long time. Sarutobi, the Sandaime Hokage, had suggested that I leave the village, when I had the strength to do so. He'd managed to convince all of Konoha that I'd perished, when he himself had nursed me back to enough health that I could travel.
After I had the strength to move again, I fled from Konoha. I've used and perfected many Kinjutsu in my life, and I didn't want anyone to find me, and force me to teach them my techniques. I've travelled all over all of the countries in our area of the world, and I've learned many things, and become even stronger than I was when I left. I'll post more specific changes to myself at a later time. All that you in Konoha need know is that I've returned, and I did so because Konoha is going to need my strength very soon.