When the relativistically strong laser pulse interacts with an underdense plasma the laser absorption depends on the laser amplitude, incidence angle, laser polarization and plasma density profile. For moderate laser intensity and weal plasma density inhomogeneity the laser pulse absorption happens at the plasma resonance region at the critical plasma density surface. This process is accompanied by the charged particle acceleration. In the case of sharp vacuum-plasma interface, the laser loses the energy via the “vacuum heating” process. It also results in the charged particle acceleration and generation of high order harmonics via the relativistic oscillating mirror mechanism as well as in the generation of the surface waves.