Maritime flag signalling, generally flag hoist signalling, is the principal means other than radio by which ships communicate to each other or to shore (distinguished from flags showing nationality, ownership, or (for naval vessels) organizational status). Virtually all signalling by non-naval vessels is now organized under the International Code of Signals (whether by flag hoist, semaphore, signal lamp, or other means), which specifies a standard set of flags and codes. Naval vessels generally use an extended set of flags and their own codes.

Source: Wikipedia