FASER, the ForwArd Search ExpeRiment, is an experimentdedicated to searching for light, extremely weakly-interactingparticles at CERN's Large Hadron Collider (LHC). Such particlesmay be produced in the very forward direction of the LHC'shigh-energy collisions and then decay to visible particles insidethe FASER detector, which is placed 480 m downstream of the ATLASinteraction point, aligned with the beam collisions axis. FASER alsoincludes a sub-detector, FASERν, designed to detect neutrinosproduced in the LHC collisions and to study their properties. Inthis paper, each component of the FASER detector is described indetail, as well as the installation of the experiment system and itscommissioning using cosmic-rays collected in September 2021 andduring the LHC pilot beam test carried out in October 2021. FASERhas successfully started taking LHC collision data in 2022, and willrun throughout LHC Run 3.