  FASER, the ForwArd Search ExpeRiment, is an experiment dedicated to searching
for light, extremely weakly-interacting particles at CERN's Large Hadron
Collider (LHC). Such particles may be produced in the very forward direction of
the LHC's high-energy collisions and then decay to visible particles inside the
FASER detector, which is placed 480 m downstream of the ATLAS interaction
point, aligned with the beam collisions axis. FASER also includes a
sub-detector, FASER$\nu$, designed to detect neutrinos produced in the LHC
collisions and to study their properties. In this paper, each component of the
FASER detector is described in detail, as well as the installation of the
experiment system and its commissioning using cosmic-rays collected in
September 2021 and during the LHC pilot beam test carried out in October 2021.
FASER will start taking LHC collision data in 2022, and will run throughout LHC
Run 3.
