Temperature and magnetic field dependence of microwave absorption in single crystals and ceramic samples of high-Tc superconductor YBa_2Cu_30_7-δ, is studied systematically. Magnetic field dependence of microwave absorption is measured at various temperatures on the same absolute absorption scale. Ceramic sample, contrary to the monocrystal, exhibits two absorption regions: low magnetic field region (up to 10 mT) dominated by the processes in intergranular medium and high magnetic field region (above 10 mT) dominated by the processes in superconductive grains, respectively. Microwave absorption phenomena in low magnetic field region are explained by the network of Josephson junctions. All the phenomena observed by both direct and modulated microwave absorption in first and second harmonic lock-in detection are well described by this model. Magnetic field dependence of microwave absorption in the high field region is ascribed to the viscous damping of vortex motion. The model of effective complex conductivity for the mixed state of type II supercoductors is proposed for the explanation of temperature and magnetic field dependence of the surface resistance. The model is used for determination of the upper critical field H_c2 for the ceramic sample at temperatures above 77 K. It is shown that the modulated absorption signal exhibits an unusual hysteresis, which has no analogon in the direct absorption signal. The hysteresis originates from the anharmonicity of the pinning force.