We report the final optical identifications of the medium-depth ( \sim 60 ksec ) , contiguous ( 2 deg ^ { 2 } ) XMM-Newton survey of the COSMOS field . XMM- Newton has detected \sim 1800 X-ray sources down to limiting fluxes of \sim 5 \times 10 ^ { -16 } , \sim 3 \times 10 ^ { -15 } , and \sim 7 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } Â in the 0.5-2 keV , 2-10 keV and 5-10 keV bands , respectively ( \sim 1 \times 10 ^ { -15 } , \sim 6 \times 10 ^ { -15 } , and \sim 1 \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } , in the three bands , respectively , over 50 % of the area ) . The work is complemented by an extensive collection of multi-wavelength data from 24 \mu m to UV , available from the COSMOS survey , for each of the X–ray sources , including spectroscopic redshifts for \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 50 % of the sample , and high-quality photometric redshifts for the rest . The XMM and multiwavelength flux limits are well matched : 1760 ( 98 % ) of the X-ray sources have optical counterparts , 1711 ( \sim 95 % ) have IRAC counterparts , and 1394 ( \sim 78 % ) have MIPS 24 \mu m detections . Thanks to the redshift completeness ( almost 100 % ) we were able to constrain the high-luminosity tail of the X–ray luminosity function confirming that the peak of the number density of logL _ { X } > 44.5 AGN is at z \sim 2 . Spectroscopically-identified obscured and unobscured AGN , as well as normal and starforming galaxies , present well-defined optical and infrared properties . We devised a robust method to identify a sample of \sim 150 high redshift ( z > 1 ) , obscured AGN candidates for which optical spectroscopy is not available . We were able to determine that the fraction of the obscured AGN population at the highest ( L _ { X } > 10 ^ { 44 } erg s ^ { -1 } ) X–ray luminosity is \sim 15 - 30 % when selection effects are taken into account , providing an important observational constraint for X–ray background synthesis . We studied in detail the optical spectrum and the overall spectral energy distribution of a prototypical Type 2 QSO , caught in a stage transitioning from being starburst dominated to AGN dominated , which was possible to isolate only thanks to the combination of X-ray and infrared observations .