The era of gravitational-wave astronomy began with Advanced LIGO's detection of GW150914. This signal has been identified as originating from the merger of a ~36 solar mass and a ~29 solar mass black hole to form a ~62 solar mass black hole at a redshift of ~0.09. In this talk, I will explain how we infer source parameters from gravitational-wave signals, and discuss the results of this analysis for LIGO's first set of observations. Gravitational waves give us a new means of exploring the properties of black holes, and i will briefly highlight some of the astrophysical implications of these measurements.