On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a gravitational-wave transient (GW150914); we characterize the properties of the source and its parameters. The data around the time of the event were analyzed coherently across the LIGO network using a suite of accurate waveform models that describe gravitational waves from a compact binary system in general relativity. GW150914 was produced by a nearly equal mass binary black hole of masses $36_{-4}^{+5}{M}_{\odot }$ and $29_{-4}^{+4}{M}_{\odot }$; for each parameter we report the median value and the range of the 90% credible interval. The dimensionless spin magnitude of the more massive black hole is bound to be $<0.7$ (at 90% probability). The luminosity distance to the source is $410_{-180}^{+160}\mathrm{Mpc}$, corresponding to a redshift $0.09_{-0.04}^{+0.03}$ assuming standard cosmology. The source location is constrained to an annulus section of $610{\mathrm{deg}}^2$, primarily in the southern hemisphere. The binary merges into a black hole of mass $62_{-4}^{+4}{M}_{\odot }$ and spin $0.67_{-0.07}^{+0.05}$. This black hole is significantly more massive than any other inferred from electromagnetic observations in the stellar-mass regime.