We present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7 in the white-light photometric band G of Gaia. The Gaia Data Processing and Analysis Consortium (DPAC) processed the raw measurements collected with the Gaia instruments during the first 14 months of the mission, and turned these into an astrometric and photometric catalogue. Gaia DR1 consists of three parts: an astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the Hipparcos and Tycho-2 catalogues (the primary astrometric data set) and the positions for an additional 1.1 billion sources (the secondary astrometric data set). The primary set forms the realisation of the Tycho-Gaia Astrometric Solution (TGAS). The second part of Gaia DR1 is the photometric data set, which contains the mean G-band magnitudes for all sources. The third part consists of the G-band light curves and the characteristics of 3000 Cepheid and RR Lyrae stars observed at high cadence around the south ecliptic pole. The positions and proper motions in the astrometric data set are given in a reference frame that is aligned with the International Celestial Reference Frame (ICRF) to better than 0.1 mas at epoch J2015.0, and non-rotating with respect to the ICRF to within 0.03 mas yr^-1. For the primary astrometric data set, the typical standard error for the positions and parallaxes is about 0.3 mas, while for the proper motions the typical standard error is about 1 mas yr^-1. Whereas it has been suggested in Gaia Collaboration et al. (2016a) that a systematic component of ∼0.3 mas should be 'added' (in quadrature) to the parallax uncertainties, Brown (2017) clarifies that reported parallax standard errors already include local systematics as a result of the calibration of the TGAS parallax uncertainties by comparison to Hipparcos parallaxes. For the subset of 94000 Hipparcos stars in the primary data set, the proper motion standard errors are much smaller, at about 0.06 mas yr^-1. For the secondary astrometric data set, the typical standard error on the positions is 10 mas. The median standard errors on the mean G-band magnitudes range from the milli-magnitude level to 0.03 mag over the magnitude range 5 to 20.7. The DPAC undertook an extensive validation of Gaia DR1 which confirmed that this data release represents a major advance in the mapping of the skies and the availability of basic stellar data that form the foundation of observational astrophysics. However, as a consequence of the very preliminary nature of this first Gaia data release, there are a number of important limitations to the data quality. These limitations are documented in the Astronomy & Astrophysics papers that accompany Gaia DR1, with further information provided in this documentation. The reader is strongly encouraged to read about these limitations and to carefully consider them before drawing conclusions from the data. This Gaia DR1 documentation complements the peer-reviewed papers that accompany the release in a Special Issue of Astronomy & Astrophysics. The papers form the primary documentation for the data release and they are frequently referenced throughout the text.