An approach based on the collective phenomena is suggested to explain how an ECR plasma is sustained by an incident electromagnetic wave. The main idea is that the incident electromagnetic wave is converted into plasma waves near the resonant surface. The plasma waves penetrate into the core ECR plasma and heat it during their nonlinear evolution. Two types of ECR plasmas are discussed: an 'underdense' ECR plasma used in the multiply charged ECRIS and an 'overdense' ECR plasma widely used in plasma assisted manufacturing and surface treatment techniques.