Ouran High School is a school for the extremely wealthy or talented. Haruhi just happens to be more talented than wealthy. Since she had a hair mishap the week before school started and could not afford the school uniform, she was mistaken for a boy.

Wandering into the Third Music Room looking for a quiet place to study, she encounters the school's Host Club — a group of attractive young men who give their time and affections to females, for a fee. After she breaks an ¥8,000,000 vase belonging to the host club, the leader demands that she work off the money by becoming a host. And when they find out Haruhi is not actually a boy, humour ensues.

[Source: AniDB]