CARMENES stands for Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs. CARMENES took 6 years from a concept to the start of operations, and a couple more years of initial data collection until the first science publication, but now is revolutionizing our knowledge on exoplanets and their stars in our immediate vicinity. Here we describe what CARMENES is: (i) an ultra-stabilized two-channel spectrograph at an almost dedicated 3.5 m telescope in southern Spain that covers in high spectral resolution and without big gaps from 0.52 μm to 1.71 μm; (ii) a science project aimed at comprehensively searching for and studying planetary systems with nearby, bright, M-dwarf hosts, but that also investigates transiting planets around other stars; and (iii) the German-Spanish consortium that designed and built the instrument and that has operated it under guaranteed and legacy time observations.