We highlight the power of the Gaia DR2 in studying many fine structures of
the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different
HRDs, depending in particular on stellar population selections. We do not aim
here for completeness in terms of types of stars or stellar evolutionary
aspects. Instead, we have chosen several illustrative examples. We describe
some of the selections that can be made in Gaia DR2 to highlight the main
structures of the Gaia HRDs. We select both field and cluster (open and
globular) stars, compare the observations with previous classifications and
with stellar evolutionary tracks, and we present variations of the Gaia HRD
with age, metallicity, and kinematics. Late stages of stellar evolution such as
hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also
analysed, as well as low-mass brown dwarf objects. The Gaia HRDs are
unprecedented in both precision and coverage of the various Milky Way stellar
populations and stellar evolutionary phases. Many fine structures of the HRDs
are presented. The clear split of the white dwarf sequence into hydrogen and
helium white dwarfs is presented for the first time in an HRD. The relation
between kinematics and the HRD is nicely illustrated. Two different populations
in a classical kinematic selection of the halo are unambiguously identified in
the HRD. Membership and mean parameters for a selected list of open clusters
are provided. They allow drawing very detailed cluster sequences, highlighting
fine structures, and providing extremely precise empirical isochrones that will
lead to more insight in stellar physics. Gaia DR2 demonstrates the potential of
combining precise astrometry and photometry for large samples for studies in
stellar evolution and stellar population and opens an entire new area for
HRD-based studies