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Cosmic shear is the small, statistically correlated distortion of distant galaxy images by weak gravitational lensing from intervening large-scale matter inhomogeneities. An individual galaxy has an unknown intrinsic shape, so the signal is measured through ensemble shape correlations rather than by assuming every source is circular. The angular and redshift dependence of these correlations probes the projected matter fluctuations, their growth and the angular diameter distance relation, constraining dark matter, cosmological density parameters and dark energy. Gravitational lensing responds to all gravitating matter rather than just luminous tracers. Intrinsic alignments and shape-measurement biases must be modeled separately from the lensing signal; the physical and statistical basis is described in arxiv.org/abs/1411.0115 .