Frankfurt is Germany's financial capital. Every other house seems to be a bank (such as the European Central Bank) - therefore the nickname Bankfurt, or insurance company, the rest are the offices of airlines and restaurants, snack bars and hotels.
And it is also famous for its trade fairs, especially the Frankfurt Book Fair.
From what I've seen during that short stay: There are lots of cultural institutions along the Main, also known as Museum embankment, such as the Staedel, the Museum for Modern Art and the Schirn Gallery. I'm definitely going there a third time, this summer. Especially since I've not even tried the famous "Ebbelwoi" (apple wine, comparable to cider).