  On September 14 2015, the LIGO interferometers captured a gravitational wave
(GW) signal from two merging black holes (BHs), opening the era of GW
astrophysics. Five BH mergers have been reported so far, three of them
involving massive BHs ($\gtrsim{}30$ M$_\odot$). According to stellar evolution
models, such massive BHs can originate from massive, relatively metal-poor
stars. The formation channels of merging BH binaries are still an open
question: a plethora of uncertainties affect the evolution of massive stellar
binaries (e.g. the process of common envelope) and their dynamics. This review
aims to discuss the open questions about BH binaries, and to present the
state-of-the-art knowledge about the astrophysics of BHs to non-specialists, in
light of the first LIGO-Virgo detections.
