The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey aimed at obtaining spectral information and redshifts of a complete sample of about 550 objects to K_sleq20.0 over two independent fields with a total area of 52 arcmin^2. In this paper we discuss the scientific motivation of such a survey, we describe the photometric and spectroscopic properties of the sample, and we release the $K_s$-band photometric catalog. Extensive simulations showed that the sample is photometrically highly complete to K_s=20. The observed galaxy counts and the R-K_s color distribution are consistent with literature results. We observed spectroscopically 94% of the sample, reaching a spectroscopic redshift identification completeness of 92% to K_sleq20.0 for the observed targets, and of 87% for the whole sample (i.e. counting also the unobserved targets). Deep spectroscopy was complemented with multi-band deep imaging in order to derive tested and reliable photometric redshifts for the galaxies lacking spectroscopic redshifts. The results show a very good agreement between the spectroscopic and the photometric redshifts with <z_{spe}-z_{phot}>=0.01 and with a dispersion of sigma_{Delta z}=0.09. Using both the spectroscopic and the photometric redshifts, we reached an overall redshift completeness of about 98%. The size of the sample, the redshift completeness, the availability of high quality photometric redshifts and multicolor spectral energy distributions make the K20 survey database one of the most complete samples available to date for constraining the currently competing scenarios of galaxy formation and for a variety of other galaxy evolution studies.
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