On September 14, 2015, the two LIGO detectors simultaneously observed a transient gravitational-wave signal, which was named GW150914. The signal matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the merged remnant black hole. The significance of the observation was determined to be greater than 5.1σ. The initial black hole masses were estimated to be 36 and 29 solar masses, and 3 solar masses were radiated in gravitational waves. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger. I describe the mechanics of this detection, which represents of culmination of 50 years of experimental and theoretical efforts; I further discuss the astrophysical implications of GW150914, and the tests of general relativity that can be performed with the signal. Last, I comment on the import of GW150914 for the field of gravitational-wave detection, and for current and future efforts to observe gravitational waves with dedicated space missions and with pulsar timing arrays.