The present paper proposes a model of relaxer ferroelectrics, which describes the polarization response of these materials in terms of vibration of the interphase boundary between the polar regions and nonpolar matrix. The model is formulated and is applied to explain various features of the polarization response of a "classical" relaxer, PbMg1/3Nb2/3O3, such as the relaxation dispersion of the linear response, its temperature dependence, and the nonlinear effects due to large ac and/or de fields.