The Pierre Auger Observatory is designed to study UHECR above 10^18 eV with unprecedented precision. The Southern Hemisphere site, built in Malargue Argentina, is taking data stably since January 2004 and it has already reached the largest exposure with respect to all the previous air shower experiments. By adopting an "hybrid" concept it combines two independent detectors: the Surface Detector, covering 3000 km^2 with an array of 1600 water Cherenkov stations, and the Fluorescence Detector, composed of 24 telescopes overlooking the array; thus allowing cross-calibration between the different techniques. The obtained results and the astrophysical implications of the measurements of energy spectrum, arrival directions and mass composition of cosmic rays above 10^18 eV will be presented and discussed.