I slept only two nights in Osaka, and one day was devoted to Nara so I just visited the city only for one morning and an afternoon - at least photography-wise - having to go back to Tokyo right after. Unfortunately, the weather was not good either...
Anyway, Osaka is the third largest city in Japan (the first two being Tokyo and Yokohama), and it's located at the mouth of the Yodo River, in the Kansai region of the main island of Honshu. Both Kyoto and Nara are less than one hour train from it. Osaka is likely the biggest and most important japanese port on the west coast.
It is traditionally known as the commercial capital of Japan, and also the national center of the gourmet food.
From the brief impression I had, it looked like it lacked (say this 50 times in a row :P) some character if compared to the "strong personalities" of Tokyo and Kyoto, suggesting me that this city, nevertheless alive and likely rich of culture and entertainment possibilities (and I can barely confirm, good food too), could be actually nice to live in.