  We review the multiple population (MP) phenomenon of globular clusters (GCs):
i.e., the evidence that GCs typically host groups of stars with different
elemental abundances and/or distinct sequences in photometric diagrams. Most
Galactic and extragalactic clusters exhibit internal variations of He, C, N, O,
Na, and Al. They host two distinct stellar populations: the first population of
stars, which resemble field stars with similar metallicities, and one or more
second stellar populations that show the signature of high-temperature
H-burning. In addition, a sub-sample of clusters hosts stellar populations with
different heavy-element abundances. The MP origin remains one of the most
puzzling, open issues of stellar astrophysics. We summarize the scenarios for
the MP formation and depict the modern picture of GCs and their stellar
populations along with the main evolutionary phases. We show that the MP
behavior dramatically changes from one cluster to another and investigate their
complexity to define common properties. We investigate relations with the host
galaxy, the parameters of the host clusters (e.g., GC's mass, age, orbit), and
stellar mass. We summarize results on spatial distribution and internal
kinematics of MPs. Finally, we review the relation between MPs and the
so-called second-parameter problem of the horizontal-branch morphology of GCs
and summarize the main findings on the extended main-sequence phenomenon in
young clusters.
