The General Systems Vehicle (GSV) is the largest class of Culture ship: less a craft than a mobile nation — kilometres in scale, home to millions or billions of people, capable of building almost anything, and entirely the expression of a single Mind's character.
Smaller classes range from the all-purpose General Contact Unit, the Culture's everyday envoy, down through a discreet hierarchy of faster, sharper vessels whose names grow euphemistic in proportion to their armament. Power for all of it comes from The Energy Grid. See Ship Minds and Avatars.