Circles (サークル saakuru) are groupings of idols in Ensemble Square. They appear to be informal gatherings of individuals who have similar interests, similar to clubs.
Clubs (部活 bukatsu) allow students to hone their skills and form bonds with students who share their same interests. In Yumenosaki, it is mandatory for every student to participate in club activities.
Committees (委員会 iinkai) are a way for students to get involved in the hands-on work related to idol performances, as well as make connections within the industry related to that committee's field. In Yumenosaki, committee activities are optional.
See also: Circle Appearances for stories pertaining to certain circles, and Club Appearances for stories pertaining to certain clubs and committees. See the Yumenosaki Private Academy page for more information about school life in Yumenosaki.