Spin waves (SW's) in a spin-polarized quantum Hall (QH) system are purely electronic excitations and represent spin-flip related intra-Landau-level collective modes where the change of total spin numbers is delta S=delta S_z=-1. These belong to the kind of lowest energy excitations in a QH ferromagnet and have a of order q^2 energy dispersion when two-dimensional wave vector q is much smaller than the inverse magnetic length. In the integer QH regime (at fillings nu=1,3,...) spin wave (SW) is effectively a two-particle excitation, i.e. a spin-exciton consisting of an electron on the upper spin sublevel of the half-filled Landau level and an effective hole remaining on the lower one. The spin-exciton spectrum can be calculated in the first order approximation in terms of parameter r_s which is the ratio of the characteristic electron interaction energy to the cyclotron energy. SW relaxation processes in QH ferromagnets are determined by different relaxation channels. The relaxation occurs through two types of interactions: (i) by an interaction which does not conserve the spin number; and (ii) by an interaction responsible for dissipation of energy, because every SW possesses at least the Zeeman energy. Analysis reveals that there are two relevant mechanisms changing spins --- the spin-orbit coupling of electrons confined to a two-dimensional channel and the hyperfine coupling to lattice nuclei. There are four dissipation mechanisms: (i) by effective Coulomb interaction of SW's renormalized to an effective dipole-dipole interaction and leading to the dynamic SW-SW scattering channel; (ii) by the kinematic SW-SW scattering in the case of the hyperfine coupling; (iii) by the kinematic SW-SW scattering provoked by electron-disorder interaction which is effective in the case of spin-orbit coupling; and (iv) by electron-phonon interaction leading to the dissipation due to phonon emission (absorbtion). There are thereby several relaxation channels responsible for diverse physics of the relaxation and for crossovers between different relaxation processes depending on magnetic field, temperature, and other parameters. As a result, SW relaxation times in QH systems range between several nanoseconds and tens of microseconds. Recent experiments have been carried out under conditions where the spin-orbit coupling and the dynamic scattering process should be predominant. The measured relaxation times of order 10ns --- are in agreement with those predicted by the theory.