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{\^}+5M_⊙ and 29_-4{\^}+4M_⊙; 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  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  deg{\^}2, primarily in the southern hemisphere. The binary merges into a black hole of mass 62_-4{\^}+4M_⊙ 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.