  The current generation of flagship X-ray missions, Chandra and XMM-Newton,
has changed our understanding of the so-called "cool core" galaxy clusters and
groups. Instead of the initial idea that the thermal gas is cooling and flowing
toward the center, the new picture envisages a complex dynamical evolution of
the intra-cluster medium (ICM) regulated by the radiative cooling and the
nongravitational heating from the active galactic nucleus (AGN). Understanding
the physics of the hot gas and its interplay with the relativistic plasma
ejected by the AGN is key for understanding the growth and evolution of
galaxies and their central black holes, the history of star formation, and the
formation of large-scale structures. It has thus become clear that the feedback
from the central black hole must be taken into account in any model of galaxy
evolution. In this paper, we draw a qualitative picture of the current
knowledge of the effects of the AGN feedback on the ICM by summarizing the
recent results in this field.
