The Culture is a post-scarcity, effectively anarchist interstellar civilisation: no money, no laws in the ordinary sense, no state to obey. What holds it together is conversation rather than government — and the principal participants in that conversation are its Minds.
Two facts shape everything else. A Mind is an individual, not a tool, and cannot be owned; and the Culture's politics are largely the politics of how a few million god-like intellects get along with one another and with the trillions of people in their care.
See also: Contact and Special Circumstances, Subliming, Infinite Fun Space.