  Within the standard framework of structure formation, where clusters and
groups of galaxies are built up from the merging of smaller systems, the
physical properties of the intracluster medium, such as the gas temperature and
the total X-ray luminosity, are predicted to possess well defined self-similar
scaling relations. Observed clusters and groups, however, show strong
deviations from these predicted relations. We argue that these deviations are
unlikely to be entirely due to observational biasses; we assume they are
physically based, due to the presence of excess entropy in the intracluster
medium in addition to that generated by accretion shocks during the formation
of the cluster. Several mechanisms have been suggested as a means of generating
this entropy. Focussing on those mechanisms that preheat the gas before it
becomes a constituent of the virialized cluster environment, we present a
simple, intuitive, physically motivated, analytic model that successfully
captures the important physics associated with the accretion of high entropy
gas onto group and cluster-scale systems. We use the model to derive the new
relationships between the observable properties of clusters and groups of
galaxies, as well as the evolution of these relations. These include the
luminosity-temperature and luminosity-$\sigma$ relations, as well as the
temperature distribution function and X-ray luminosity function. These
properties are found to be a more accurate description of the observations than
those predicted from the standard framework. Future observations will that
further test the efficacy of the preheated gas scenario are also discussed.
