A group of seven high schoolers who entertain classmate clientele for a fee.
Ouran High School's Host Club was founded by Tamaki Suou, then a first year high school student as a place for it's members to express themselves freely. His first conscripted member was Kyouya Ootori, who Tamaki convinced didn't need to be relegated to a secondary position amongst his brothers if he was willing to put forth the effort required to surpass them. Next came Mitsukuni Haninozuka, who wished for a place to freely enjoy cute things and sweets, as well as his constant companion Takashi Morinozuka. Finally, came the Hitachiin twins, Kaoru and Hikaru, who are free to be their mischievous selves (and perhaps eventually learn their own differences). The Hitachiins were in middle school at the time of their enlistment, the club did not
The final addition to the Host Club was Haruhi Fujioka, who was drafted into the group by virtue of her smashing an $80,000 vase intended to fund raise for the club.
Each customer designates a host. Most customers designate a permanent regular host who entertains them whenever they visit the club-- although some customers jump from host to host, or occasionally one host will take over for another. The customers pay for the services of their hosts, although it is unclear how much. Tthey also occasionally buy merchandise (which ranges from a photo calendar to the personal item of a host).
The club frequently has themed days. During which time the music room is redecorated to suit the theme and the hosts all wear themed costumes as well. The redecoration and costumes are generally expensive; 90% of the money the club receives from customers goes back into these events.
While not all of the members have an official role in the club, most have a designated role in Tamaki's fantasy "family."