The basic concepts related to the phenomenon of field line stochastization through external magnetic perturbations, e.g., resonant magnetic surfaces, magnetic islands, overlap of island chains, stochastic instability, Kolmogorov length, field line diffusivity, etc., are discussed. Different models for description of transport of heat, background and impurity particles in stochastic magnetic field are presented. Stochastization impacts on the plasma rotation, radial electric field and electromagnetic fluctuations in the plasma are described. Mechanisms responsible for mitigation of the edge localized modes, ELMs, are introduced.