  Since the first detection of gravitational-wave (GW), GW150914, September
14th 2015, the multi-messenger astronomy added a new way of observing the
Universe together with electromagnetic (EM) waves and neutrinos. After two
years, GW together with its EM counterpart from binary neutron stars, GW170817
and GRB170817A, has been observed. The detection of GWs opened a new window of
astronomy/astrophysics and will be an important messenger to understand the
Universe. In this article, we briefly review the gravitational-wave and the
astrophysical sources and introduce the basic principle of the laser
interferometer as a gravitational-wave detector and its noise sources to
understand how the gravitational-waves are detected in the laser
interferometer. Finally, we summarize the search algorithms currently used in
the gravitational-wave observatories and the detector characterization
algorithms used to suppress noises and to monitor data quality in order to
improve the reach of the astrophysical searches.
