CHEOPS (CHaracterizing ExOPlanet Satellite), the first S-class mission (small) of ESA's Cosmic Vision 2015-2025, is dedicated to the search for exoplanet transits by means of very high precision photometry of bright stars already known to host planets. Launched on December 18 2019, the first science data are expected in the coming weeks. CHEOPS will provide the unique ability to determine accurate size for a subset of planets in the mass range from super-Earth to Neptune. By combining masses and size, it will be possible to providing key information on the small planet composition and internal structure. By identifying transiting exoplanets with a high potential for in-depth characterization - e.g. those that are potentially rocky and have thin atmospheres - CHEOPS will also provide primary objectives for future instruments suitable for the spectroscopic characterization of exoplanetary atmospheres. I will illustrate the instrument, its performance, the science objectives of the Guaranteed Time Observation program, and the opportunity for the community offered by the Guest Observing program.