Successful detections of gravitational waves (GWs) opened a new window to explore the Universe and launched a new era of multimessenger astronomy. Three observing runs completed so far by LIGO-Virgo-KAGRA detectors, resulted with a catalog of 90 events, mostly (but not exclusively) comprising coalescing binary black holes (BHs). These observations of compact binary coalescences present an unprecedented view of the properties of black holes and neutron stars.
In my talk I will introduce the basics of GW physics and astrophysical sources of GWs - both these currently being detected and those expected to be detected in the near future. Since the ongoing observations have revealed the existence of binary BH systems, we started our journey to the world of these mysterious objects. Hence, I will also introduce to non-specialists the essence of a black hole, followed by description of astrophysical scenarios of their formation. Lessons we learned so far regarding the properties of BHs will be summarized. Once popular, but now forgotten idea of white holes will also be discussed.