It's the third largest cities in Japan, while Yokohama is the second. The history of Osaka is rather interesting, since it was created just for commercial purpose by Toyotomi Hideyoshi (1536-1598), who forced local merchants to move there. During that time Hideyoshi needed commercial center apart from Kyoto the Imperial city. Osaka is also known as "water city" which name came from it's numerous water canals.
Four cities within Osaka, Kobe, Kyoto and Nara forms a circle of Kansai area. It has it's own culture, quite different from from Kanto (Tokyo) area's. People are said to be more open and easy-going, which might be formed from it's commercial background.
Osaka has it's own castle comparable to Tokyo's Edo castle. The main tower of Osaka castle has 8 floors while the most top one is the observation deck and 7th the home of Hideyoshi Toyomi. The main tower was renovated in Heisei period.
Another interesting feature of Osaka is that it's only city in Japan which has it's own museum dedicated to culture of comedy. Osaka is the birth place of manzai, a stand up comedy performed in pairs.