  Galaxy groups and poor clusters are more common than rich clusters, and host
the largest fraction of matter content in the Universe. Hence, their studies
are key to understand the gravitational and thermal evolution of the bulk of
the cosmic matter. Moreover, because of their shallower gravitational
potential, galaxy groups are systems where non-gravitational processes (e.g.,
cooling, AGN feedback, star formation) are expected to have a higher impact on
the distribution of baryons, and on the general physical properties, than in
more massive objects, inducing systematic departures from the expected scaling
relations. Despite their paramount importance from the astrophysical and
cosmological point of view, the challenges in their detection have limited the
studies of galaxy groups. Upcoming large surveys will change this picture,
reassigning to galaxy groups their central role in studying the structure
formation and evolution in the Universe, and in measuring the cosmic baryonic
content. Here, we review the recent literature on various scaling relations
between X-ray and optical properties of these systems, focusing on the
observational measurements, and the progress in our understanding of the
deviations from the self-similar expectations on groups' scales. We discuss
some of the sources of these deviations, and how feedback from supernovae
and/or AGNs impacts the general properties and the reconstructed scaling laws.
Finally, we discuss future prospects in the study of galaxy groups.
