  Charge order has recently been identified as a leading competitor of
high-temperature superconductivity in moderately doped cuprates. We provide a
survey of universal and materials-specific aspects of this phenomenon, with
emphasis on results obtained by scattering methods. In particular, we discuss
the structure, periodicity, and stability range of the charge-ordered state,
its response to various external perturbations, the influence of disorder, the
coexistence and competition with superconductivity, as well as collective
charge dynamics. In the context of this journal issue which honors Roger
Cowley's legacy, we also discuss the connection of charge ordering with lattice
vibrations and the central-peak phenomenon. We end the review with an outlook
on research opportunities offered by new synthesis methods and experimental
platforms, including cuprate thin films and superlattices.
