  After giving a brief introduction and presenting a complete classification of
gravitational waves (GWs) according to their frequencies, we review and
summarize the detection methods, the sensitivities, and the sources. We notice
that real-time detections are possible above 300 pHz. Below 300 pHz, the
detections are possible on GW imprints or indirectly. We are on the verge of
detection. The progress in this field will be promising and thriving. We will
see improvement of a few orders to several orders of magnitude in the GW
detection sensitivities over all frequency bands in the next hundred years.
