This extensive article was written under the guidance of FZU group and it describes the world largest Fluorescence detector system. The group initiated the very idea of writing the paper, contributed with a chapter on the optical setup, organization of various international groups, text editing, correspondence with referees, chapter on detector operation, etc. The article describes the components of the fluorescence detector including its optical system, the design of the camera, the electronics, and the systems for relative and absolute calibrations. The operation and the monitoring of the detector, the system performance and precision of shower reconstructions are also discussed. The paper is the main collaboration reference. Scientists from 17 countries contribute to the Pierre Auger Observatory. The observatory is built in Argentina, covering 3000 km2 and operating 27 fluorescence telescopes, 15 of which contain segmented mirrors designed and built by the staff of FZÚ.
