Ouran Academy is a prestigious school for the ultra-rich in Japan. Families from all areas of private and public sector business send their children there, to learn what a rich child who will one day inherit his family business should learn. These rich children have quite a lot of money and quite a lot of free time, and so, the girls (at least) have been given a wonderful way to relax… and part with their ludicrously huge allowances.
The Ouran High School Host Club was the brainchild of the half-French Suoh Tamaki, who considers it his duty to entertain women. It's… not really the same thing as a proper host club — most of the Hosts are under age after all — and mostly serves to treat girls to tea, cakes, and eye candy.
The Host Club gets up to all sorts of activities — weekends away, festivals, dances and even stranger things have been done, all in the name of entertaining the womenfolk. The Host Club is incredibly popular thanks to this, and has hundreds — perhaps thousands — of regulars. They don't all come in at the same time, of course… Well, usually.